"Calling the pilot of space ship X911!" Evan Harwich shouted into the radio transmitter of his little Interplanetary Patrol Boat. "Good God! Turn your crate back, you crazy fool! Don't you know you're headed right into the danger zone of Jupiter's Forbidden Moon? You'll get yourself burned to a crisp in another few seconds if you don't turn back...."

Evan Harwich's growling voice was almost shrill at the end. His police duties patrolling the vicinity of Io, innermost of Jupiter's larger satellites, rarely developed moments as tense as this. Most other pilots had brains enough to give the Forbidden Moon a wide berth. And for excellent if mysterious reasons!

Yet the craft ahead, a sleek new job with the identification number X911 painted on its conning tower, kept steadily on. Its slim hull, which betrayed an experimental look, was pointed straight at the threatening greyish disc of Io, the one world in the solar system which no exploring ship of the void had ever reached—intact!

Almost everybody among the inhabited spheres knew about the dangers of the desolate Forbidden Moon. Ever since the colonial empire of Earth had been extended to the region of Jupiter and his numerous satellites, Io had been a grim menace; sure destruction to any rocket that approached within five thousand miles of its dreary, almost airless surface.

Nobody seemed to know just why this was true; but some scientists claimed that somehow there was an invisible layer or shell all around Io; an immense blanket of strange energy or force that fused and blasted the metal hulls of all ether craft that ran into its insidious web.

Tensely and helplessly Evan Harwich watched, as the ship ahead continued on its way toward what seemed sure catastrophe. No danger in front of the recklessly piloted craft could be seen, of course. Five thousand miles of clear, cold vacuum was all that was visible between it and Io. But since this region held concealed in it all the potential violence of a hair-triggered trap, ready to unleash a flaming death that involved unknown physical laws and principles, maybe it wasn't just plain vacuum after all!

With dogged persistence Harwich kept yelling futile warnings into his radio. His shouts and curses were unheeded, and no answer was given. He knew what was going to happen in another second. There would be a burst of dazzling white fire all around the rocket of this foolhardy pilot he had tried to save from suicide. Metal would drip and sparkle in the absolute zero of space. In just another instant....

Harwich swung his patrol boat aside, not caring to end his own life. But he kept watching the X911 from the side-ports of his cabin.

And now, something quite different from what he had expected was taking place. Suddenly the apparently doomed ship was enveloped in a bluish halo which seemed to emanate from a great helix or spiral of metal that wrapped its hull!

Immediately afterward, as the X911 entered definitely into the zone of destruction around Io, great white sparks lanced dazzlingly through the blue halo. It was as though the latter was fighting back those gigantic, unknown forces that had seemed to make the Forbidden Moon forever inviolable. It was as though the halo was keeping the X911, and whoever was flying it, safe!

Evan Harwich's slitted eyes widened a little in astonishment and hope. "Dammit!" he grumbled happily. "That idiot's got some kind of new invention that's protecting him! Maybe the Forbidden Moon is going to be reached and explored after all!"

A second more that weird conflict of hidden forces continued. Watching it was like watching a race, on which you have staked everything you own. Visibly, that daredevil space ship seemed to slow, as if resisted by a tangible medium. For an agonizing instant of suspense, Harwich saw those wicked sparks brighten in the X911's bluish aura. Then the latter dimmed, flickered, went out!

As if angry demons were waiting to pounce, destruction struck—quicker than a lightning bolt.

If there had been any humor in the situation before, it was gone now utterly! The patrol man's lips dropped apart in sheer awe. The muscles of his massive, freckle-smeared forearms tightened futilely as he longed to help the X911's doomed pilot. In the pit of his stomach there was a sickish feeling.

Where that rocket that had dared the inscrutable enigma of the Forbidden Moon had been, there was a sudden, terrific blaze of light. The intolerable incandescence of it seemed to reach out to infinity itself, illuminating even the blackness between the distant stars of space. But it was all as silent as the bouncing of a bubble on velvet. No explosion, however huge, can transmit sound in the emptiness of the void.

The magnificent, horrible blast broke into a million gobs and sparks of molten metal—from what had once been a space ship's hull. Superheated gas from ignited rocket fuel shot out. Scattered far and wide, the white-hot fragments of the wreck continued on their way, following the original direction of the once bold X911 toward Io. Their speed increased gradually, as the gravity of the Forbidden Moon pulled them. The larger chunks, falling at meteoric speed, would bury themselves deep in the cold Ionian deserts.

The secret of Io had claimed another victim, one who might have been victorious. But Io's mystery was still unviolated. Evan Harwich had seen other ships, disabled and unmaneuverable for some reason beforehand, go to their ends like this; but he was still not used to the spectacle, and to the unholy wonder it provoked in him.

Dazzled and almost blinded, he guided his patrol boat shakily away from the Forbidden Moon. There was cold sweat in his thick, black hair, under his leather helmet; and cold sweat too on his narrow, bristly cheeks. His movements of the controls were a trifle vague and fumbling with emotion, making his patrol boat waver a little in its course.

For perhaps the millionth time Harwich wondered: "What makes Io so dangerous? Dammit all, those scientists who claim that there is a deadly shell of unseen energy completely enveloping the Forbidden Moon, must be right! There isn't anything else that could explain the continual destruction of all rocket craft that come within that five-thousand-mile limit!"

Evan Harwich was ready to accept this much as fact. But beyond this, there was still a vast, unguessable question mark.

Was this shell of energy a natural phenomenon; or was it something planned, made, intended for a purpose? If the latter guess was right, who could have created such a gigantic screen of force? What kind of beings? What kind of science?

Io was an almost dead world, Harwich knew. Very cold. Very little water and air. Astronomers had taken photographs of its terrain through powerful telescopes, from the other moons of Jupiter. Very little could be seen on those photographs but deserts and grey hills, and curious formations which might be the magnificent ruins left by an extinct race.

Evan Harwich was far from a weakling; but cold chills were playing over his big body as he groped to understand the unknown.

His vision was clearing somewhat, after having been so dazzled by the incandescent blast that had accompanied the destruction of the X911 a moment ago.

In the feeble sunlight, so far out here in the void, Harwich saw a second rocket, leaving the scene of the disaster along with himself. Evidently someone else had witnessed that weird demonstration of Io's destructive might, too!

Squinting through a pair of binoculars, Harwich read the obviously ancient craft's number. Then he snapped on his radio again.

"Calling space ship RQ257!" he grated into the transmitter. "Interplanetary Patrol just behind you. Pilot, please identify yourself! Do you know who was aboard the experimental rocket X911, that was just destroyed?"

A few seconds later he heard a dazed, grief-anguished voice speaking in response: "Yes ... I ought to know. I came out to watch our test of the Energy Barrage Penetrator, which we thought would be successful. I am Paul Arnold. The man who was just killed was John Arnold, my father."

John Arnold! Yes, Harwich had often seen photographs of this daring, hawk-faced old student of the Forbidden Moon in the scientific journals. He had been the greatest of them all! But there wasn't much to do for him now but shrug ironically, and report the nature of his death by radio to the Interplanetary Patrol Base on Ganymede, largest of Jupiter's satellites.

"I'm sorry, Paul Arnold," the patrol man told his informant in sincere sympathy.

"Thank you," the quavering voice of Paul Arnold returned. "And now, if you don't mind, I've got to get back to Ganymede City. Dad's gone, but I've got to carry on his work."

Harwich didn't meet Paul Arnold, the son of the dead scientist, face to face for more than a month, Earthtime. But on patrol duty out there in the lonely reaches of the void, with the stars and the roar of his rocket motors for company, he saw a good deal of the leering, greyish sphere of Io. It seemed to taunt him with its masked secrets, hanging so near to the tremendously greater bulk of Jupiter. But the Forbidden Moon told him nothing new at all. Through his binoculars he saw the deserts and hills and those supposed ruins. Near the equator was something that looked like a vast, pointed tower. But Harwich had seen this before, often. Something moved near the tower now and then, as on other occasions. But maybe this distant movement was only the shifting of clouds of dust, blown by a thin, frigid wind, in a tenuous atmosphere.

Then, back in Ganymede City, came that meeting with Paul Arnold. It happened at the Spacemen's Haven. Evan Harwich, on furlough now, was sipping Martian kasarki at the bar.

Presently a hand was laid on his arm. He turned to face a slight-built youngster, who could not have been more than eightteen. But his peculiar gold-flecked eyes were as distant and scared and bright as if they had seen Hell itself.

"You're Harwich," said the boy. "I'm Arnold. They pointed you out to me as the patrol pilot who reported my father's death. I wanted to talk to you. I don't know just why, except that you were there too, when Dad was killed. You saw what happened. And people have told me that you were a square shooter, Harwich."

Somewhat startled, but glad to know the youth, and more than willing to talk with him on the subject mentioned, Evan Harwich tried to smile encouragingly. It wasn't too easy, considering his weathered, space darkened features and threatening size; but he did his best.

"Pleased to meet yuh, Arnold," he said rather clumsily, offering a big hamlike hand. "I wanted to talk to you too. How about a drink and a quiet corner, where the crowd here won't be stepping all over us?"

They retired to a table in a screened nook. "Now," said young Arnold, "you've seen as much of the Forbidden Moon as anybody alive, Harwich. You must know that the energy aura around her is real and not a fable. You must know, too, that it couldn't be a natural phenomenon, since nothing in nature acts like it does. There's only one alternative possibility as to what could cause it! Even though Io seems so deserted, somehow there are machines there, functioning to maintain that shell of force! Right?"

Harwich nodded. Little glints of intense interest seemed to show in his eyes. "I've believed that for a long time," he admitted. "But those machines must be plenty wonderful to build up a barrage of invisible energy, thousands of miles in extent! Our scientists couldn't even begin to dream of doing anything like it! Even the principles employed must be a million years ahead of our time!"

"Right again!" the boy responded. For a second he cast a guarded, suspicious glance around the room, where Earthmen and leathery Martians were talking and laughing and drinking.

"The evidence can't be disputed," Paul Arnold whispered at last. "It might be that the people who invented those machines have been extinct for ages. But the mechanisms they created are still operating. There's superscience there on Io, Harwich! How much could we benefit civilization, if we could somehow find out what the principles of those machines are? How much damage might be done if those principles happened to fall into the wrong hands, among men? War and conquest—a whole solar system thrown into chaos—might result!"

Evan Harwich wanted to laugh scornfully, wanted to call the kid a dreamer of wild dreams; but the realization that young Arnold probably told the truth, made his hide tingle and pucker instead.

"Maybe you're right, fella," he growled.

"Of course I am!" Arnold almost snapped. "My father believed it for years, and his work must go on, even though the Forbidden Moon scares me plenty. You saw yourself, Harwich, that his Energy Barrage Penetrator was almost successful. I've been trying to build another, with enough power to get through."

Harwich's lips curved, a nameless, wild thrill stirring in his blood. But after all, even before he'd left a great consolidated farm in southern Illinois nine years ago, to become a spaceman, he'd been an adventurer at heart.

"Do you suppose you'll need any help?" he asked simply, realizing that even as he spoke, death on a tomb-world might well be lurking in the background.

The question sounded like impulse, but it wasn't. Harwich had lived too long in the shadow of the Forbidden Moon's taunting enigma, not to want to take a personal part in any effort to penetrate its grim secrets. Besides, he had a month's furlough from patrol duty now. The thought of possible adventures to come made his nerves tingle.

Paul Arnold's eyes widened. "I almost hoped you would want to join me, Harwich," he stammered happily, seeming only to need the moral support of an experienced spaceman, to bring him out of the black mood he was in. "Shall we go to my laboratory?"

The Arnold lab and dwelling proved to be one of the oddest that Evan Harwich had ever seen. It was just outside the great steel-ribbed airdrome that confined a warm, breatheable atmosphere over Ganymede City, the small mining metropolis of a dying world.

The Arnold lab was a group of subterranean rooms, beneath the desert. They were reached by a private tunnel from the City, and were hermetically sealed against leakage of air to the cold semi-vacuum of the Ganymedean atmosphere above.

Cellar rooms, vaults, not exactly modern but restored from some ancient ruin; for Ganymede had had its extinct clans of quasihuman people too, ages ago. A weird place, this was, a place of poverty, perhaps, since all of the Arnold resources must have gone into experimentation; but a homey sort of place, too, with its scatterings of books and quaint art objects and pictures.

"This is the Energy Barrage Penetrator, Harwich," Paul Arnold was saying in husky tones, as the two men bent over a copper helix or spiral, attached to a maze of wires, tubes, and power-packs. "I rebuilt it here on this test-block from Dad's plans; with certain rearrangements, of course. But we need a new Gyon condenser, if we want to raise the Penetrator's strength enough to make our venture successful."

Evan Harwich nodded beneath the single illuminator bulb that glowed here, its rays glinting from the battered, patched hull of the space ship, RQ257, that stood in the center of the great room, under the airtight exit doors provided for it in the ceiling.

"So I see," Harwich commented with subdued eagerness. "Well, that's not so bad. I can buy a new Gyon condenser from one of the supply shops in town. I'm no scientist, fella, but they give us a pretty complete scientific training in the patrol service. Enough so that I can see that the Penetrator is going to do the trick, this time, with your improvements. And I don't think it will take very long to get things ready for a real trip to the Forbidden Moon."

The patrol man had hardly finished speaking, when a door, somewhere, groaned on its hinges. In the dusty silence there were footsteps, coming nearer through the series of rooms.

"Well, have we got company?" a voice boomed heavily after a moment.

Evan Harwich turned about slowly. Standing in the arched entrance of the laboratory chamber, beneath the ancient, grinning gargoyle of carven granite that formed the keystone of the arch, were two people. They must have just come in from town.

One was a man, as tall as Harwich himself, but much broader. He looked jovial, overfed, and just faintly sly. Harwich knew him a little. He kept a small printer's establishment in Ganymede City, repaired delicate instruments, and made loans on the side.

"Hello, Harwich!" the big man greeted loudly. "You look surprised to see me here! Well, I'm just as up in the air as you are, to find you around. How come? You see I've been financing Paul Arnold's researches since old John was killed. Has Paulie talked you into some part in the great miracle hunt on Io, too?"

"Hello yourself, Bayley," the patrol man returned in not too friendly a tone. "Yes, I've joined up."

Harwich was a little more than surprised to see the fat printer here. He didn't like the setup at all. Not that he had anything definite against George Bayley. The latter had always seemed good-natured and honest, except for some elusive trace of insincerity in his manner, his voice, and his little squinted eyes.

Was this the kind of man for Paul Arnold to choose as a patron, particularly when he was in pursuit of the incredibly advanced science which must exist on Io? A science that might benefit the human race immeasurably, or might result in wholesale destruction and confusion, if it was wrongly and selfishly used?

Evan Harwich couldn't have answered yes or no to this question.

There was a painful pause in the conversation. Harwich found himself looking at the girl, who had entered with the big printer, and to whose arms the latter clung with a kind of bearish possessiveness. She was small and dainty. Her blonde hair, combed back tightly, fitted her head like a cap. She was wearing a plain but tasteful black dress with a white collar.

"Oh, I'm sorry!" Paul Arnold exclaimed after a moment. "Clara, this is Evan Harwich of the Patrol. Evan, this is my sister. I didn't tell you that I had a sister, did I?"

The girl only nodded slightly, and smiled a warm, friendly little smile. But why did the big patrol pilot find her more attractive than any other girl he had ever seen? Perhaps mostly it was those wistful eyes of hers, not gold flecked like her brother's, but clouded amber. They were mild and troubled and knowing. Maybe Clara Arnold's life, as the daughter of a martyred scientist, had made them like that. Harwich knew that he might conquer not only the Forbidden Moon, but the stars themselves, and still remember those eyes.

"Now we all know each other," Bayley boomed. "We're one big happy family—or are we?" He looked at Harwich significantly, a definite scowl now crinkling his heavy brows. "Harwich," he added, "we appreciate your company a lot. Only we are engaged in some pretty serious business here, and it doesn't allow us to take in outsiders."

For reasons of his own, Bayley was trying to get rid of the big patrol pilot. But Harwich was inclined to be very stubborn, naturally, and faint, pleading looks from both Clara and Paul Arnold, made him doubly so, just at present.

Harwich had the aspect of a very dangerous adversary in a physical encounter; his weathered features were far from beautiful, and at certain times he had a way of grinning that made him look like a good-natured devil with a hot pitchfork hid behind his back. He turned on that grin, now.

"What's in that package sticking out of your coat-pocket, George?" he asked the fat printer breezily. "It's about the right size and shape to be the new Gyon condenser we need. I was going to buy one myself; but seeing that you've already done so, we might as well go to work installing it in the Penetrator apparatus."

"Well, all right, Harwich," Bayley growled with some slight show of timidity. "As long as you're Paul's friend, I suppose you can stick around."

"Thanks a lot, George," Harwich chuckled, as the printer set the package containing the precious Gyon condenser on a work table.

The patrol pilot was almost sure he heard faint sighs of relief from the two Arnolds, as Bayley backed down. Had they come to mistrust him too, since he had been financing them? Did they feel more at ease because he, Evan Harwich, whom Bayley could never bulldoze, was their partner now too?

The spaceman wondered, and he couldn't help wondering something else. On Clara Arnold's left hand, there was a diamond gleaming. An engagement ring. Bayley's? The way the latter had clung to the girl's arm, it couldn't very well be anybody else's. Could Clara, quiet and beautiful, ever love the boisterous, paunchy printer?

The Arnolds were a strange family, anyway. The son was ready to sacrifice his life in an effort to reach the Forbidden Moon, where his father's ashes lay entombed. The daughter? Might she not be of the same fanatical breed? Might she not be willing to marry Bayley, so that he would supply funds for their experiments?

For a moment, Evan Harwich felt a sharp, hurt ache, deep in his heart. But he fought it down. All this was none of his business. And from a heavy-glazed window slit in the ceiling of the laboratory room, a shaft of soft light from ugly Io, the Forbidden Moon, was stabbing down, appealing to his own adventurous nature.

Paul had slipped on a pair of lab coveralls. He tossed another pair to the patrol pilot. "Come on! Let's get started, Evan," he urged pleasantly. "We've got a big job in front of us, and remember you said we'd get through with it before long!"

True to Harwich's predictions, the rearrangement of the Energy Barrage Penetrator for far greater power than the original had possessed, did not take really a lot of time.

Within forty hours after the patrol pilot's arrival at the lab, the task of installing the Arnold apparatus in the old space ship, RQ257, was complete. The tests of the Penetrator had been made, and judged as successful as anyone could have hoped for.

The space ship stood ready there in the laboratory room, a slender, copper helix wrapped around its hull.

"All set, eh?" George Bayley boomed jovially. "Got your emergency supply-packs loaded aboard, too, eh? But you won't need them, boys," he added seriously. "You've got everything in your favor. And in five hours you'll be back here with Clara and me, at the lab with a dandy story to tell."

Bayley seemed honest and sincere, now. Evan Harwich almost felt sheepish about the matter. Maybe he'd misjudged the big, bearish printer. Anyway, he watched his every move, during the assembly and installation of the Penetrator.

Paul Arnold was whistling a little tune of confidence and exultation. Harwich's pulses beat happily, his thoughts on the enigma of the Forbidden Moon, that now must yield to the new Energy Barrage Penetrator. Superscience there on Io! Unutterable wonders! Who could guess beforehand what the Forbidden Moon's vast screen of force was meant to bar from intrusion? But maybe they would soon know!

Only Clara Arnold showed worry. There was a slight shadow in her amber eyes, when she took Harwich's hand.

"I suppose this is only a preliminary test flight to Io and back," she said. "Not much dangerous exploration. But please be careful," she pleaded. "Please be careful, Evan."

The spaceman muttered a word of thanks. Evan. His first name. To have Clara Arnold use it like that might have given a new meaning to life. His heart was suddenly pounding very hard, before he remembered that diamond on her left hand. She was promised to George Bayley.

The girl and the printer retreated from the laboratory chamber, waving a farewell. The space ship was sealed. The great exit doors in the ceiling of the lab opened wide, and the air rushed out.

In another moment the RQ257 was shooting skyward. In the night, among the welter of stars, huge Jupiter and his many satellites shone down on the Ganymedean deserts. The nose of the ship swung unerringly toward Io.

The RQ257, wrapped in its protecting halo of blue fire from the Penetrator, struck the Forbidden Moon's tremendous, invisible envelope of energy, squarely. There was a snarling sound in the ship's interior. White sparks lanced through cold space beyond the windows of the cabin, as two opposed forces fought each other. But the RQ257 bored on steadily.

"We're going to make it, Paul!" Harwich shouted through the reeking, dinning cabin.

"Of course we are!" young Arnold yelled back at him. "How could we fail!"

The two men were on the brink of success.

Then there was an abrupt, strident, angry, snap from the vitals of the Penetrator apparatus. Everything seemed to happen at once. The protecting blue aura outside the ship waxed and waned perilously. And whenever it waned, there was a grinding, crumpling sound, as of steel plating being crushed like so much paper in a giant's grip. Heat, and the cindery pungence of scorched metal, filled the cabin.

Paul Arnold and Evan Harwich were frozen rigid with stunning, agonized paralysis, as strange energy snapped into their bodies. In the jolting, erratic motion of the wounded space ship, the two men were hurled from their feet like a pair of stiff wooden dolls.

Rolling and tumbling, his vision half blinded, Harwich saw the metal walls of the cabin buckle and redden with heat, as the craft floundered in that region of mysterious force and energy that heretofore had destroyed every ship that had attempted to reach Io.

There was another growl from the protecting apparatus. In a flash of electricity, the side of the bakelite case that housed the Gyon condenser exploded outward. At once the staggering Penetrator quit completely. Its last shred of protecting force was gone.

But that momentary hell had ended, too, with almost dazing suddenness. The grinding, snapping sounds had ceased. And there was only the heat and the stench of burnt metal, and the weightless sensation of free fall. That and the mocking stars.

Paul Arnold, panting, his face darkened and beaded with perspiration, clutched a bakelite handrail in one corner.

"We got through Io's energy barrage!" he shouted wildly. "We did that much, at least; and for a moment, when our Penetrator went wrong, I didn't think our luck would be even that good."

Evan Harwich leered back at the youth, from near the now useless apparatus that John Arnold had invented. "Yes, we got through," he grunted hoarsely. "The energy shell must be only a couple of thousand miles thick, with free space underneath, between it and Io itself. The Gyon condenser kept working raggedly just long enough to get us out of the danger zone, without being completely blown apart!"

Harwich didn't have to test the controls of the ship to know that they were useless, now. The rockets were silent too. The RQ257 was falling free toward the Forbidden Moon, still a couple of thousand miles beneath.

"But dammit, Evan!" young Arnold growled. "The Gyon condenser shouldn't have quit on us at all! Those things are tested for heavy loads of power!"

The patrol pilot was well aware of that. Clinging to the base of the Penetrator, he was close enough to see detail. The lights in the cabin had gone out, but the ugly effulgence of Io was streaming through the windows.

Projecting from the shattered bakelite box of the Gyon condenser, were two slender, bent wires that should have been joined together. It had been one wire once, but it had snapped in the middle.

The ends were faintly scorched and blued; but there was something else, too. They were bevelled off curiously, as if they had been notched.

"Cut with a file!" Harwich fairly snarled. "The wire was cut with a file. Then the insulation was rewrapped carefully so that all the evidence was hidden!"

The cause of the accident was plain. The wire had been able to carry the load of power easily enough during the tests; but under the additional load of fighting the Ionian hell-zone, it had burned through and snapped!

"Bayley!" Paul Arnold whispered in the ominous stillness that now pervaded the plummeting derelict of the RQ257. "He brought the condenser, you remember! Evan, I know you were careful to watch everything he did during the assembly and tests in the lab itself. He must have had the Gyon condenser at his apartment before he brought it to us. He must have doctored it there! He was planning even then to get rid of me! And when he found you around, he decided that he wouldn't weep if he got rid of you too!"

"But why?" Harwich growled in momentary confusion. "Why should Bayley want to get rid of you?"

It was almost a silly question, as Harwich realized at once; but now Paul was answering it.

"It's simple," said the youth. "Bayley financed me after Dad was killed—yes. He watched my experiments and tests and studied my apparatus. He has a pretty keen mind. With me out of the way, no one but himself will know just how the Penetrator works! He can fix up another ship and come to Io himself without any competition! Anything he learns or discovers on the Forbidden Moon will be his alone! Or so he thinks, anyway."

It was too clear now! Evan Harwich knew that he and the boy were tumbling helplessly into the maw of hell now. In a useless, derelict ship they were falling toward the Forbidden Moon! They were already within the gates of unholy mystery! Death seemed very close. Yet the cold anger that hissed in the patrol pilot's brain, made him determined to live, somehow, for revenge!

"We'll be smashed if we stay in the ship, Paul," he said fiercely. "So we've got to jump for it with our safety equipment."

Quickly and more smoothly than did the youth, for he was well-trained, Harwich got into his space armor. Next he donned two massive packs, one on his chest and one on his back.

The exit door of the cabin was jammed, but with his pistol the patrol pilot fired an explosive bullet into its hinges.

A second afterward, Arnold and Harwich crept through the rent, while escaping air puffed out around them. They leaped into the emptiness almost together. With the heat-warped wreck of the gallant old RQ257 falling beside them, they continued their plummeting descent. There were still almost a thousand miles to go, for the distance between Io itself, and the gigantic energy envelope that surrounded it, was perhaps three thousand miles.

Down and down, with only regulation spacemen's emergency equipment to rely on to avert being crushed on those greyish hills and deserts, rushing nearer and nearer. Even a thousand miles did not take many moments at that terrific speed.

The Forbidden Moon was like a sullen, silent nether world, with an atmosphere so rare that an unprotected human being would gasp and die in it in a few minutes! Even a man in a space suit could not hope to survive that desolation for long! Io seemed like a Pit now to Evan Harwich, an Abyss of Hell from which there was no escape! A place where no Earth being was meant to venture!

This moment was too grim to think of thrills. Helplessness removed that intriguing glamor utterly. And there was only savage determination left. That and smoldering hate of the man who had caused misfortune!

Presently, through the thin metal of his oxygen helmet, Harwich heard a soft, hissing, whistling sound. Gradually it grew stronger. The patrol pilot knew what it was, of course. He had entered the intensely thin upper atmosphere of Io, and the hissing was made by his own space armored body passing through those tenuous gases at fearful velocity.

The sound served as a signal for action. Again, though the situation was new to him, Harwich's training made his responses accurate. With a gauntletted hand, he groped for the metal ring on the pack that bulged from his chest. It was ancient history when he jerked that ring, but sometimes, in emergency landings like this, on worlds that had a blanket of air, however slight, it was still useful. In another second the patrol pilot was dangling beneath a gigantic mushroom of metal fabric. He felt the firm tug of the shrouds. Deceleration.

He wondered vaguely why the fragile parachute did not tear apart in the terrific speed of his fall. But it was the utter thinness of the air, of course, here in the upper layer. Its resistance was so very slight. So there was time for velocity to be checked gradually, as the air grew denser, and its retarding effect greater with lowered altitude.

Paul Arnold had opened his chute too. Its vast top, a hundred feet in diameter, gleamed dully in the faint sunshine.

In a great plume of dust far below, the derelict space ship crashed. Fire flew as the force of the impact generated heat. But the wreckage was out of sight, and there was only a pit smoldering on a bleak, dusty hillside. The RQ257 was buried deep.

Harwich and Paul Arnold landed several miles away from the grave of the ruined ship; for they had drifted with the thin, dry, frigid wind.

Their booted feet spanged painfully against the sand and broken rock, and they crumpled to their knees; for even in the feeble gravity of Io the impact had been heavy.

Harwich snapped on his helmet radio-phone. Young Arnold's voice was already audible in it, faint and thready and sarcastic.

"Well, here we are, Evan," he was saying. "The first Earthmen to set foot alive on the Enchanted World! I guess I got part of what I wanted anyway, didn't I? But with what equipment we've got to keep alive with, we might just as well be buried with the RQ257! Funny I'm not scared. I guess I don't realize...."

His bitterly humorous tone faded away in vague awe.

Still lying prone the two men, looked around them, at the hellish, utterly desolate scene. The hills brooded there under the blue-black sky and tenuous, heatless sunshine. A rock loomed up from a heap of sand. It was a weathered monolith with weird carvings on it, resembling closely those left by the extinct peoples of Ganymede, that other, now colonized moon of Jupiter. A curious pulpy shrub, ugly and weird, grew beside the monolith. A scanty breath of breeze stirred up a little ripple of dust.

That and the stillness. The stillness of a tomb. Harwich could hear the muted rustle of the pulses in his head. Everything here seemed to emphasize the plain facts. The Forbidden Moon was a trap to them now. A pit from which they could expect no rescue. An abyss that was worse than the worst dungeon—worse than being literally buried alive!

It was like the end of things. Was this the kind of slow, creeping, maddening death that George Bayley, the treacherous printer, had planned for them?

Again fury steadied Evan Harwich's determination. Grimly he struggled to steady his nerves.

"Listen, Paul," he said quietly into his phones. "We mustn't ever let ourselves think we're licked! That's sure poison! The stuff we've got in our emergency packs will enable us to keep living for a while anyhow. We know Bayley'll come to Io sometime, with a ship fitted out with a new Penetrator. We know he'll be looking for the secret of the force aura of the Forbidden Moon, and whatever else there is to find. Maybe we can get ahead of him yet, if we keep on the move. Which way do you suppose would be best to go?"

Harwich asked this question because Paul Arnold, in his more academic study of Io, should know more about its terrain than he.

"You know the Tower?" Paul Arnold questioned. "The queer pinnacle, or ruin, or building, near the equator, on what is known as the Western Hemisphere? You must have seen it often when you were on patrol."

Harwich nodded. He remembered very well. Only a hundred hours ago, still on duty as a patrol pilot, he'd seen that pointed mystery from the void, vague dusty movement around its base.

"It was my Dad's guess that whatever miracles are to be discovered on Io, they will probably be located around the Tower," Paul Arnold answered. "But I was careful to notice our position when we landed. We're far north of the Tower now—a good fifteen hundred miles. A nice, long walk—especially when the normal air of the Forbidden Moon is too thin to be breatheable."

"Stop that pessimist stuff, and let's get started!" Harwich snapped. "We'll have to live very primitively, of course, but who knows what will turn up?"

They discarded their parachutes and started out, plodding southward, carrying their heavy packs. As if to save their energy, they did not speak much.

The hills rolled past, under their plodding feet. More fragmentary ruins appeared, and were left behind. Their boots sank into soft dust, as they marched on and on. At first their muscles were fresh, but tiredness came at last. And the miles which lay ahead were all but undiminished.

The tiny sun sank into the west and the cold increased. Night was coming.

"We'd better camp," young Arnold suggested wearily.

So they opened their packs, and took out the carefully folded sections of airtight fabric that composed their tent. It was part of the usual equipment kept for emergency purposes by those in danger of being stranded on dead or almost dead worlds. The tent could be hermetically sealed. Harwich and Arnold set it up carefully and crept inside. Air was freed from their oxygen flask, and the queer shelter ballooned out like a bubble.

They could remove their space suits now, and breathe, here in the tent. They ate sparingly from their concentrated rations. Meanwhile a little pump and separator unit, driven by a tiny atomic motor, was busy compressing the thin Ionian air, separating out the excess of carbon-dioxide and nitrogen it contained, and forcing the oxygen into the depleted air flasks.

Once in the darkness Paul and Evan were awakened by a strange sound, eerie in that dead quiet, and very faint because the scant Ionian atmosphere could not conduct it well. But when they crept to the flexoglass window of the tent, they saw nothing unusual.

"I guess we're getting jumpy," Paul whispered nervously, his breath steaming in the cold, frosty air that filled the shelter.

"It looks that way," Evan Harwich returned reassuringly.

But after the boy was asleep again, he crept back to the frosted window to watch. He knew that there had to be something mighty on Io. The shell of force that surrounded the evil moon couldn't exist all alone. There had to be more. Something that lay back of it, went with it. Something that could easily be very dangerous.

Jupiter, so near to Io, was a gigantic threatening mass in the heavens. But its light was deceptive. There were so many dense shadows.

Did he see some of the stars near the horizon wink out suddenly, and then appear again, as though something big and nameless and sinister had momentarily blocked their light and then passed on? He could not be sure, and nothing further happened. To save his companion unnecessary concern, when nothing could be done about the threatening danger anyway, he decided to keep the incident to himself.

Long before the dawn they were once more on the march. How many hours was the Ionian day? Something over forty. It didn't matter much.

When the daylight finally came, they had slept again, this time in their space suits, without bothering to set up the tent. Rising to his feet, Paul Arnold pointed suddenly.

"Look! An ancient road!" he shouted.

It was true. The highway ran there between the hills. A stone ribbon, covered here and there with drifted sand, which showed that there was no traffic of any sort now. The ruins along it looked a little less battered than those which the two men had previously seen, and there were vast lumps of corroded metal, too. Machinery in a former age.

"The road goes our way," Harwich commented. "We'll follow it."

Hours later, Paul Arnold offered an opinion. "Part of the mystery of Io is clearing up, Evan," he said. "The ruins around here. They're almost identical in architecture to the ruins of Ganymede and the other Jovian satellites. The evidence looks plain. There must have been a single great civilization once, extending over all the moons of Jupiter."

Harwich, thinking of, and hating George Bayley for his diabolical treachery, was only half listening.

"Yes?" he questioned.

"Yes," the boy answered. "And look at those dry ditches, and the big, rusty pumps! The valley here must have been rich, irrigated farmland, once!"

They were going across a huge bridge, now, made of porcelain blocks. It was a magnificent structure, magnificently designed according to intricate principles of engineering.

"What I can't understand is why all this country became deserted," Paul offered. "You'd think that people who could build things like this would never die out! They could conquer any difficulty that might come up, it would almost seem. Even if their world got old and worn out. After all, even Earthmen can make almost dead worlds artificially habitable again with airdromes, and with imported atmosphere and water."

This was another mystery. But it touched Evan Harwich's thoughts only faintly. Nor did he care very much when later Paul pointed out to him rich deposits of ore—outcroppings along the road. He'd seen them himself, and the tunnel mouths, too, of ancient mine workings. There were many fortunes to be won here, in costly metals, just as on the other Jovian satellites. But how could this be important, now, with death dogging their tracks, and so many other things more important, to be concerned with?

Evan Harwich reserved his determination for what he knew was coming. The slow wearing down of stamina. Water he and Paul had a little of. And more could be reclaimed from the thin, dry atmosphere. It collected in the bottoms of oxygen bottles, when they were pumped full, condensed by compression. A few precious drops. You could drink it out after each bottle was emptied of air. Just about enough water to sustain life.

In the matter of food, you had to ration yourself so stringently that you caught yourself looking with longing eyes at the few, weird, bulbous shrubs and the scattered lichens, which were the only vegetation on this dying world. Only you knew that these arid growths would never be good to eat.

Those long Ionian days passed. One after another. Five, ten, fifteen. Harwich knew he was losing strength slowly. The inevitable was catching up with him. But those hard years in the Interplanetary Patrol Service, and the rigid physical discipline, had made him as tough as steel wire.

With the boy, Paul Arnold, it was not the same. He was very young, and not too robust. And he was slipping fast.

"What's the matter with me, Evan?" he would grumble. "All this desert isn't real, is it? We're not on the Forbidden Moon, are we? I'm dreaming."

"You're just tired out, that's all, fella," Harwich would answer in a tone that he would try to make reassuring. He would put an arm around the kid's shoulders, to support his faltering steps.

Big brother stuff.... Paul had plenty of pluck, all right, but there wasn't much else left in him. He was wearing out, mile by mile, staggering under his heavy pack.

Every resource was reaching its limit, now. Food supplies had dwindled away to nothing, at last. The little atomic motor that worked the air compressor and separator unit, was breaking down. It could hardly pump enough oxygen into the air flasks any more.

But there was nothing to do but keep on the march, anyway, in spite of handicaps. Evan Harwich felt as though he was going slowly mad. Brooding thoughts came into his mind constantly.

Clara Arnold. Where was she now? What had happened back there on Ganymede? What had George Bayley done? When would he come to Io, with the ship he would surely fit out with a new Penetrator?

What was Clara thinking? What if she knew her brother was alive on the Forbidden Moon, but slowly dying? What if Bayley told her that maybe Paul was still alive, adding that he himself was the only person that might be able to effect a rescue? What if he had finally used this means, this possibility, to make Clara marry him? She didn't love Bayley, the fat printer! She couldn't! And he wouldn't even have to promise to attempt a rescue—only suggest that he might try. Clara must be half crazy herself, thinking of her brother. After all she'd lost her father to the Forbidden Moon too.

The thought of demure Clara Arnold in the arms of that bulky, squint-eyed printer, who had shown his true colors at last, and proved his diabolical cleverness, fairly strangled Harwich. Maybe he had no right to harbor such an attitude. After all he hardly knew Clara. He only knew her haunting beauty and friendly amber eyes, with quiet wisdom and a little of the martyr in them—like her father, perhaps. But Harwich couldn't help thinking. It was only by exercising super-human self-control, that he kept himself from turning into a raving maniac.

Supporting Paul Arnold's feeble, struggling steps, Harwich watched the sky like a starved, wounded wolf. Sometimes, in sheer, wild determination, he longed to claw at that cold, forbidding firmament, and climb out of that hell-pit of a world into which he had fallen. He yearned with a savagery beyond words to claw his way up there into space, to wherever George Bayley might be, and feel the fat throat of the man who had tampered with the Gyon condenser aboard the RQ257, squeezed between his hooked fingers.

But the frigid sky and the bleak, dying hills, and the weary miles, mocked all his hate-born desires. His numbed, aching feet could only plod on and on in this grave-like desert. Ruins, rusted machinery, silence, and cold that crept even through the heavy insulation of his space armor.

Still, he could remember another thing. In the far distance to the south, was something wonderful and strange. Something that made the deadly and insidious energy barrier of the Forbidden Moon possible. Where the Tower loomed on the astronomical photographs of Io.

That night came at last when a streak of silver fire traced its way across the sky. It couldn't be anything but the flames ejected from the rockets of an approaching space ship.

Paul Arnold saw it too, turning his haggard face upward. "There he is, Evan," he croaked into his helmet phones. "Bayley's coming at last."

"I see," Harwich returned softly; his teeth gritted and his lips curling furiously, behind the transparent front of his space headgear.

They dropped down beside the wall of a ruin, to watch. The ship was coming straight in, toward Io. At its tremendous altitude, nothing but its rocket blasts could be seen at first. But then there was a sudden flare of bluish light. It had struck Io's force barrier, and that blue glow was the evidence of a Penetrator, functioning. The craft seemed to slow a little, as its pale, protecting shell of counter-energy fought back that invisible, guardian screen of the devil moon.

"He got through the force shield," Harwich growled after a moment. "We knew he would, of course, with his Penetrator operating right. Damn him!"

There was no more blue fire visible now; but the little silver-tailed path of rocket flame, showed that the ship was coming in safe and sound, its propelling jets working steadily.

Among the stars it turned southward toward that deepest enigma of Io. Toward the unknown scientific wisdom, which lay hidden somewhere near the Ionian equator.

"He'll get there in a few minutes' time," Paul whispered. "And I guess we won't get there at all. I'm sorry, Evan, that I got you mixed up with the Forbidden Moon. Me—I'm just about finished—now."

Paul Arnold's voice trailed away. Harwich turned the boy's glass-covered face up. In the light of monster Jupiter, he could see that it was blank and relaxed. The eyes were closed. In the quiet rays of the giant of planets, the youth looked as though death had already touched him. But there was a little frosty blur on the inside of the crystalline face-plate of his helmet. It showed that he still breathed.

Tottering a little himself, Harwich picked the boy up, pack and all. He struggled to put one foot ahead of the other, marching again toward the south, where the space ship was rapidly receding. Had his strength been at normal level, his load, bulky though it was, would have been light in this weak gravity. But Harwich was near the end of his rope, too. And so he moved on through that beautiful shadow-haunted, frigid night, where no man was meant to live.

Many times he had to stop and rest. After a short while, the atomic motor of the air compressor separator unit refused to work any more. Harwich tried turning the mechanism by hand. But this was slow, exhausting work.

He watched the luminous dial of the cold-proof wrist-watch, strapped on the outside of one of his heavy space gantlets. His mind was getting dimmer. Cold was biting home, savagely. Harwich wanted to see just how much longer he could keep going. It was eight hours now, since Bayley's ship had appeared. Slowly more time crept by. His boots trudged in the desert dust, mechanically. The hands of his watch moved on. One hour more. Another.

Why didn't he desert the dead weight of Paul Arnold? But you never deserted somebody who was like a kid brother, did you?

The patrol pilot's breath was coming fast and short, now. The last of his air was being used up. It was useless to try to replenish the oxygen flasks with hand power, even though he was suffocating.

Harwich tripped in the dust, and fell sprawling. Jupiter, shining down upon him, somehow looked like a fat face, tremendously bloated in size—the face of George Bayley. Harwich cursed, and tried to crawl toward the south.

Did he hear a sound through his oxygen helmet—a sound loud enough for the tenuous Ionian atmosphere to transmit? Or was it only the roaring of the unsteady pulses in his ears? He tried to look ahead, but his vision was very dim, now, and the light of Jupiter and his moons was so confusing. The shadows of the rocks and the ruined buildings were so very black.

But suddenly Harwich squinted. Something was moving toward him, skimming low over the ground, but not touching it. Something that glinted wickedly, and showed long, shadowy arms. It was no hallucination. Evan Harwich was sure of that! Fear came out of that numb fog into which his brain was settling. It gave him a last, feeble spurt of strength. He knew that here he must be facing a tiny part of Io's colossal riddle.

He tried to crawl away from nameless danger, dragging Paul Arnold with him. He got behind a mass of million-year-old masonry, tufted with prickly plants.

But the thing that pursued him, easily overcame his weak, instinctive effort to find concealment. Cold metal claws closed on him. He felt himself lifted upward, into the night. His mind toppled away into black nothingness.

Somehow, it wasn't the end of life. Harwich began to regain his senses, slowly. First he heard a distant, muffled clanging. For a long time before he paid any real attention to the fact, he was aware that strange warm rays were pouring down upon his body. They seemed to heal and soothe his aching muscles.

He opened his eyes at last. Startled, he sat up. Around him was the warm glitter of glass and metal. His space suit was gone. He was in a crystalline cage, filled with warm, humid air. Odd gadgets, like ray lamps used in therapy, were fitted to the ceiling. Strange, tropical vegetation grew in the cage, and water tinkled somewhere.

There was a kind of soothing quiet over the place, except for that distant clanging. There was a smoothness to everything; a mood of mechanical refinement and perfection. It was almost hypnotic, somehow. It dazed and quieted the senses.

Paul Arnold, clad in the slacks and shirt he'd worn under his space armor, was lying on the floor beside Harwich. He was still unconscious, but he was breathing evenly. His color was much better than before. The rays from the roof above were slowly healing his weakened body.

Evan Harwich shook the boy gently. "Wake up, Paul!" he urged. "This must be it! The center of Power! The place we wanted to find! Some kind of machine brought us!"

Paul Arnold rubbed his eyes and sat up. Together, Harwich and the boy looked around through the crystal walls of the cage in which they were confined.

"There—there's the Tower!" young Arnold stammered at last, pointing.

It glittered in the faint morning sunshine. It was undoubtedly the same huge pinnacle that astronomers had photographed from the other moons of Jupiter. Only it was close, now, its details sharp and clear and real. Around its slender, tapered spire, thousands of feet aloft, the faintest of frosty aureoles clung; a ghostly light, like the sundogs of Earthly winter days.

"The Tower must be the source of the Ionian force envelope, Evan!" Paul Arnold offered after a moment. "That light up there at its top almost proves it."

Both men were talking vaguely, thinking vaguely, looking around vaguely. In part this must have been because of sheer wonder. Places like the Spacemen's Haven on Ganymede seemed as far away as a dream now.

An incomprehensible sense of depression was creeping over Evan Harwich, as he studied his surroundings further. There were many other cages in view, arranged in blocks, with paved alleyways between. Vegetation was thick in the evidently air-conditioned habitations. Little pools of water glistened in them daintily, strange paradox on dying Io.

And there were creatures, too. Scores of them in each cage. Strange, fragile, sluglike animals crept about aimlessly. They looked just faintly human, with their pinkish skins and manlike heads. But there was no slight shadow of intelligence in those great, sad, stupid eyes.

Harwich wasn't squeamish, but he looked at these futile animals with a certain pitying revulsion. "What kind of a nursery place have we got ourselves into, Paul?" he grumbled quizzically.

Arnold shrugged. "They're something like men, these things, aren't they?" he offered in puzzlement. "Maybe that's another unknown quantity to figure out. But this place is plenty wonderful, though. Look!"

The youth was pointing upward. Against the cold Ionian sky a flattened object was circling at low altitude. A flying machine without wings, it seemed to be. From it dangled strange webby metal arms, as it moved in a circular path, above the surrounding desert hills. It seemed to keep watch over those thousands of crystal cages in the valley. It must be a guardian of some sort.

"I'm not at all sure I like it here," Harwich growled. "We were fixed up, revived, made new men again, so to speak; but still I don't like it here."

"Somehow I've got the same idea," Paul Arnold agreed with a quizzical smile.

A little clinking noise behind the two men made them turn about. After that, awe kept them spellbound. They didn't speak. What was there to say? They didn't try to retreat, either. What was the use? If what they saw was danger, they could do nothing to avert it. Hypnotized with wonder, they only stared, feeling as helpless as the larvae in an ant-hill, tended and cared for by the workers.

A section of the cage-bottom had raised, like a trapdoor. A bulk was creeping through the opening. It was a machine, so marvelous, so refined in its functioning, that it seemed far more than alive. It was flat, like a small tractor; but there were no treads for it to move on. It seemed, rather, to glide on a cushioning, grayish mist. The thing purred softly, like a great cat, and tiny lights twinkled in crystalline parts of it—batteries to deliver fearful atomic or cosmic power, perhaps. The mechanism had many flexible tentacular arms of metal that glinted with a lavendar luster.

But even the substance of those arms, the metal itself, looked indefinite and eye-hurting at the edges, as though it was partly fourth-dimensional, or something.

Both men grasped the truth. Here was that million-year advancement of science that they'd talked about with such thrilled fascination, in the stuffy bar of the Spacemen's Haven, back in Ganymede City. But Ganymede City, with all its human crudeness and inefficiency, seemed like a lost, happy legend, now, to Arnold and Harwich. Far, far away, and dim. For here was dread wonder to eclipse it. Futurian fact! Physical principles of such a miraculous order that mankind had scarcely dreamed of their outer fringes yet, were functioning here.

The flat machine advanced. But it was only instinct working, when the two men crouched away from it a little. It was useless to fight; it was useless to run.

"Get away, you!" Paul Arnold grumbled dully to the mechanism. "Beat it! Scram."

And Harwich was reacting in a similar manner. "What the hell!" he stammered. "What are you trying to do with us."

It was almost funny—the ineffectual, confused protest of those two men. They were like children too lost in their new environment to know what was dangerous and what was not.

Misty, lavender tentacles reached out and grasped them carefully. They were lifted from the floor of the cage like babes. Once Harwich's great freckled arms tautened, as though he was going to battle the monstrous miracle that held him. But futility checked the urge. Where was there anything to win by struggling, now? And how could a mere man win anyway, against soft-moving mechanical power, that should belong to the far future? Oddly the tentacles were warm and tingling, not cold like you'd think metal should be.

And so Arnold and Harwich submitted to a paternal, mechanical dominance, regretfully, because there was nothing else to do. It hurt their sense of freedom, but where was there any alternative?

Still floating a little off the tile pavement of the cage, the machine carried the two men easily to the opening in the floor, and glided down into a crystal-roofed tunnel. There it began to accelerate swiftly, flying with bullet-like speed, a foot or so above the glass bottom of the passage.

The tunnel's roof was transparent as air. Through it, Harwich and Arnold could see that they were nearing the Tower rapidly. After only a moment of whizzing, breath-taking flight, they had arrived within that great, enigmatic edifice, for the passage entered its base.

There, in an eerie half-twilight, the flat little machine released the two humans whom it had brought here, to the Tower.

Mute with an even greater wonder than before, Harwich and Arnold stared around them. The room was gigantic, soaring up in a huge, metal-ribbed dome. Scores of crystal-walled passages led into this colossal chamber of secrets. The whole immense Tower building was transparent, except that some darkening pigment had been added to the material that composed it, 'till it was like bluish glass. Through it the desolate surrounding hills of Io could be seen, and the cages, filled with those aimless, pathetic, sluglike creatures.

But the attention of the two men was drawn inevitably to the center of the room. Rearing up there, under the rotunda of the dome, was a massive, lavender-sheened pyramid. It gave a steady, throbbing sound, as of countless tiny wheels and shafts whirling inside it, working cams and rods, and who knew what else?

"Dammit!" Evan Harwich kept muttering under his breath in dim confusion. "Dammit."

He was used to machinery, yes. He was used to the roar of rockets, and to the delicate instruments used in space flight. But this was machinery of a far higher order. That busy, vibrating pyramid, squatting there like some huge idol, somehow seemed to possess a definite personality of its own!

Suddenly Paul Arnold clutched the patrol pilot's arm. "I wonder if I believe what I see!" he whispered tensely. "Look!"

Harwich's gaze followed the lines of the boy's pointing finger to something quite near—so near, and seemingly so insignificant in this vast, somber, throbbing interior, that he had not noticed before.

Just at the base of the pyramid there was an artistic little structure, consisting of four slender pillars and a roof. It looked like a small, ornamental kiosk or arbor, so artfully were the scientific details of it—the coils in its top, and the delicate filaments that pronged from them—concealed in the decorative metal scroll-work.

Within the pillared structure, somehow, there stood a man—an Earthman. His heavy body was clad now in a rocketeer's leather coverall. At his waist dangled a heat pistol, and on his fat face there was a strange, wild sort of smirk.

"Howdy, boys!" he greeted. "Yes, it's me—George Bayley, the guy who used to keep a print shop in Ganymede City! I've been here longer than you have, and I've been able to find out more. Pretty nice, huh? The people of Io had science perfected before they became extinct. Everything was done by machines, even investing. Not a bit of work to do any more. And if they wanted anything special, they just came into this little coop, here, and wished."

Bayley paused, still smirking. His loud voice had seemed distant in that great room, and vibrant with awe. Harwich and Arnold stared at him for a moment, neither knowing quite what to say, or what to believe.

And what was that which had just spilled from his lips, as though he had been a little afraid of the statement himself? About perfected science, and wishing?

"You're crazy!" Evan Harwich stormed fiercely. "You're a liar!"

But his furious tone was tremulous with doubt, even as he spoke. He knew at once that he'd just grabbed onto these words, and uttered them, maybe because, somehow, he hated Bayley, and wanted to contradict his seemingly impossible claims. But in this temple of un-Earthly marvels, one's whole standard of judgment was upset. Possible and impossible became meaningless terms here, at the foot of this great, whirring pyramid, which seemed a symbol of omnipotence.

"Crazy?" Bayley questioned. "No, Harwich, you can't say that, when you're all tangled up and fuddled yourself! What I said about wishing is true. Telepathic control of machines, it must be. This place is so damned wonderful that it would turn Aladdin of the Wonderful Lamp green with envy! And it would drive the Genie of the Lamp down into his shoes in shame!"

Harwich's doubts, if they had been doubts, and not just confusions, began to dim a trifle. After all, one of the big objectives of the science of Earthmen, was to make life easier; to transfer as much of the burden of work as possible to machines. Why couldn't the same objective have been conceived here on the Forbidden Moon? Not only conceived, but accomplished? Io was an old world; life had begun here sooner than on Earth, and science, too! So there had been more time for advancement.

"All right, Bayley," Harwich growled grudgingly. "Tell us what you've discovered."

"Yes, for Pete sake, tell us!" Paul Arnold joined in.

It was odd, the way they were asking the fat printer for information, now, when they should be hating him for the wrongs he had done them. But, perhaps, the human mind can hold only so much at one time. For the moment there was room only for dazed awe and questioning in their thoughts, and hatred was temporarily pushed into the background. The equal of Aladdin's miracles did not seem so far from possibility, here!

"Okay!" George Bayley rumbled. "Glad to spill the beans; what I know of them. I arrived here in my space ship about fourteen hours ago, when it was still dark. The Tower building here looked by far the most important, so I came straight to it. There were machines flying about, but they paid no attention to me at all, so I wasn't worried much about what they might do to me.

"Leaving my ship on the other side of the Tower, I got into this room through a tunnel. I was wearing a space armor, of course. I passed through a kind of airlock. This chamber was just like you see it now, except that lights were burning, because it was night."

"And then?" Paul Arnold questioned eagerly.

"Exploring, I climbed into this little metal coop, here at the foot of the pyramid," Bayley went on. "By then I was pretty flabbergasted with all I'd seen. I began to think I needed a drink of something strong. Yep, it must have been telepathy! Because presto—one of those flat flying machines with the tentacles, whizzed up to me from a tunnel exit. It was carrying a kind of crystal carafe.

"Boy, I didn't know what to think! I didn't know whether I ought to taste the stuff in that carafe, at first. But finally I did. It was damned good. Not alcoholic, but something a whole lot better."

Harwich and Arnold looked at each other, as Bayley paused, as if to get his breath. They looked up at the pyramid, throbbing above them, like some great, cryptic, servant personality. The feeling that Bayley was telling the truth, was growing on them.

"Naturally you tried other things, after the carafe was brought to you, Bayley," Paul Arnold prompted. "You wanted to see how much further this expression of desires by telepathy might be carried. You wanted to see how much more you could use the ancient Ionian science."

Bayley, still standing in that little metal-pillared structure, nodded slowly. "You catch on quick, Arnold," he said. "First I wished for gold, since it was the first thing I thought of. The sounds inside the pyramid changed a little, as though an order was going out somehow, maybe by radio. Five minutes later a whole bunch of those flying machines came into the Tower here, carrying bars of gold in their tentacles. There it is."

The printer was pointing toward a dully gleaming heap of yellow ingots near the farther wall of the chamber.

"But this, I soon found out, was just kid stuff!" Bayley continued. "I suppose if I'd thought of radium here in this wishing coop, I would have got a couple of tons of that, too! But I wished for a space ship—something special, beyond anything an Earthman ever saw before! Well, the pyramid buzzed a little longer and stranger this time, as though it was sort of thinking and planning, and as though the wheels inside it were maybe inventing, too. Then, somewhere far off, there was a lot of pounding for about an hour. I guess you know the answer, boys. There she is—the sweetest little super-futuristic space flier you ever saw!"

Harwich and Arnold stared at the torpedo-like ship that rested in a cradle-like support nearby. It was completely without rocket-tubes, or other visible means of propulsion. But its rakish lines and wicked lavender glitter made it look as though it might well reach the distant stars themselves.

Evan Harwich bit his lip tensely. Suddenly a thought struck him. "Did you see any Ionians since you've been here, Bayley?" he asked. "Any living, intelligent beings who might question your right to be prowling around?"

Bayley laughed. "Not one!" he returned. "They're extinct, I'm sure of it! And that's lucky for me."

The patrol pilot was beginning to put the pieces of the Forbidden Moon's riddle together at last. And Paul Harwich must have been doing the same. The evidence, as far as it went, was clear.

Perfected science! The fat printer had told them that all you had to do was think your wishes in that queer little pillared structure. And the machines translated your wishes into fact. Unless Bayley had lied, and there was small reason to suppose that he had, the rest was maybe not so difficult to understand.

First, the great envelope of force around Io. That was to keep possibly dangerous intruders away, of course. Thus, the ancient Ionians had lived in carefree idleness and luxury, tended by their perfected machines. The thing in the pyramid must be the master servant mechanism, reachable in that pillared kiosk, by telepathy. It must be the coordinator, in contact with the other mechanisms by radio, or something. Adding and calculating machines, way back in the Twentieth Century, had thought and reasoned, after a fashion. More recently, on Earth, apparati of a similar nature had done far more, working out intricate mathematical problems, far more swiftly and accurately than any human being could.

And the apparatus within the pyramid must be much the same thing, but developed to the nth degree! A vast planning, calculating device that could reason and invent with a swiftness and perfection far beyond any living mind. But it was still just mechanical; a servant apparatus that thought by the turning of the wheels and the movement of levers inside it with no more consciousness than an adding machine of the Twentieth Century!

This was the way Harwich figured it all out. And he saw something else, too.

"Uh-uh, Bayley," he remarked suddenly. "Soon after that new space flier was brought here at your command, you decided that you were complete boss around here, didn't you? There were no ancient Ionians in your way. All you had to do was wish, inside that telepathy kiosk, and it was just like Aladdin wishing with his lamp, eh?"

For the first time, cold, comprehending anger had come into the patrol pilot's tone.

"Why sure—sure!" Bayley growled back at him. "And why not? Just about anything I can think of is possible! And, let me tell you something else, you poor dope! You and Arnold wouldn't be alive now, if I hadn't wished it! I thought you might have gotten through the Ionian force shield somehow, when the RQ257 cracked up. I thought you might be somewhere out there on the desert still living. So I just wished that the machines go and get you, and revive you if you needed it. I thought maybe it might be fun."

It was enough. Cold anger reborn in Evan Harwich's breast was suddenly rekindled into blazing fury by the memory of the RQ257, and a wire filed almost through in a Gyon condenser. Evan Harwich's muscles tightened. Wordlessly he was about to leap at George Bayley.

But a warm metal tentacle whipped suddenly about his waist. The flat mechanism that had brought him and Arnold to the Tower, had seized him. Again, he was helpless.

"You see?" Bayley drawled. "I really am boss, here, just as you said. I just wished that you be restrained, and you are! But I've been doing too much talking and explaining. How about a little showing for a change, huh?"

"Damn you, Bayley!" Harwich growled, but the fat printer ignored the curse.

He only grimaced crookedly. "Let's make a couple more wishes," he taunted. "A couple of really good ones! How about a whole fleet of space ships, for instance? The biggest, most powerful fleet in the solar system! All automatic craft, capable of flying and maneuvering unmanned! Then, let's see, the other wish? It's not so difficult either. Both you and Arnold are my deadly enemies, Harwich. I think it would be fun to make my enemies squirm a little. I'd like to see you crack up, Harwich! You've always been so tough! So how about some kind of a discomfort device? Something really special? In short, a torture instrument! Come on, pretty machines! Do your stuff!"

Paul Arnold's face turned pale, but he bit his lip courageously. Evan Harwich studied the strange, wild light in the fat printer's squinted eyes, and waited for whatever would happen.

There was a crescendoing whir within that huge pyramidal coordinator. The man who had usurped the rule of the ancient Ionians over their mechanical servitors, had given his telepathic orders. Already there were signs of obedience. Thinking and planning was going on in that pyramid; thinking and planning more intricate than that of the greatest human wizard that had ever lived, more soulless and swift than that of an adding machine.

Presently, from far away, came a thin, shrill sound. Looking back through the darkened glass walls of the Tower room, Harwich and Arnold, both of them clutched, now, by the tentacles of the flat robot, saw a horde of black specks collecting against the sky in the pale sunlight outside. A flock of those flat, tentacled, flying things.

They seemed to emerge from an opening in the ground; from a vault where perhaps they'd been stored for ages. In a gigantic swarm they hovered over the glass cages and their pathetic animal inhabitants. Then, drifting like gulls away from this weird city of the Forbidden Moon, they moved off toward the surrounding hills.

There, like swarming bees, they settled in their tremendous numbers, on the open, arid valley. Flame tools in their tendrils were brought into play. Dust, reddened with heat, began to rise.

"They're leveling the ground!" Paul Arnold whispered hoarsely. "They must be preparing a shipyard!"

"Sure, kid," George Bayley laughed, trying to conceal the half-scared wonder in his own voice. "Maybe it'll take weeks for them to build the fleet I asked for! But they'll do it! You'll see, if I happen to let you live that long!"

The unholy wizardry of the Forbidden Moon was proven beyond all doubt. And in this weird Tower room, air-conditioned against the cold thinness of the atmosphere beyond its wall, the pyramid still throbbed a shrill portent of more to come.

A second robot mechanism soared into the chamber from a tunnel mouth. It bore a curious tripod-like instrument. The flying automaton spiralled down like a bubble, and came to rest beside Harwich and the youth. Pinioned by the tendrils of the other automaton, they were helpless to do anything but watch and submit. They were pushed flat on their backs, and held firmly. The tripod instrument was set up between them.

"The discomfort device, this must be!" Bayley gloated, shifting his weight from one foot to the other. "In just a few seconds there's going to be some fun, I'll bet! Now, Harwich and Arnold, I'm wishing you bad luck. Just a little foretaste of what I might wish later! Okay, pretty machines! Give my beloved enemies the works, just for a second."

Two rods of metal, projecting down from the tripod, were set in position by one of the automatons. One rod touched Harwich's skull, the other Paul Arnold's. A switch was moved.

There was no sound; but all of the patrol pilot's body seemed suddenly and maddeningly afire. To the very center of his mind, agony stabbed, viciously. No searing pain of any injury he had ever received, could have equaled this. He writhed, longing to scream his lungs out, as that moment of sheer hell seemed to last an age.

"God!" Paul gasped when it was over.

Both men were sweating and limp, and yet no visible harm had been done to their bodies. Artificial sensation, the torture must have been. Nerve impulses transmitted directly to the brain. A devilish, perverted achievement of superscience! Such agony might conceivably go on, in Satanic refinement, for months, without bringing death.

"You see, boys, I'm boss here as long as I stay in this little telepathy coop, where the old Ionians used to give their orders!" George Bayley hissed triumphantly. "All the wonders of the Forbidden Moon are mine to use, just as I see fit! There were just a bunch of machines here, waiting for somebody to control them. A pistol doesn't ask who pulls its trigger! And I got here first!"

"I was afraid of something like this when we were still on Ganymede, before any of us knew," Paul Arnold muttered raggedly.

And Evan Harwich understood very well what the youth meant. George Bayley was feeling that touch of power here. A sense of omnipotence was flattering his shallow ego, raising him in his own estimation to the level of some ruthless god. He, who had been a petty business man, a printer, a repairer of instruments, a loan shark! Just a crumby, fat little human being, ridiculous, small and conceited. Pathetic, too, stubborn, and lacking in judgment. There were many like him on Earth, and among the scattered spheres of Earth's interplanetary empire.

Maybe, after all, the wisdom of the Forbidden Moon was too big for the human race. Maybe they would have to grow themselves first, advance in evolution, before they would know how to handle and how to win real benefits from such wisdom.

"All right, Nero," Harwich growled contemptuously to Bayley. "I'll grant that you're in the driver's seat, ready to stop nowhere. Building a space fleet and all. But where is Clara Arnold?"

The patrol pilot asked the question with fear and doubt in his heart.

"Clara Arnold?" said Bayley almost casually. "Too damned clever for a girl! Said she thought I might have had something to do with the crackup of the RQ257. Said she was worried about Paul and you, too, Harwich, being maybe stranded still alive here on Io. But she said that she'd finally decided my promises weren't good for anything, anyway. That I'd have to rescue you two men first before she'd believe in me. Until then, our engagement was off."

Harwich felt a brief wave of elation, as he heard these words. Clara had seemed so quiet and timid; but she'd evidently proved herself plenty courageous and plenty smart.

"But where is she?" Harwich growled angrily. "Now, I mean!"

"Don't get excited," Bayley sneered. "She came to the Forbidden Moon with me, hoping to see you and the kid again. I left her locked in my rocket. But she can't mean much to me any more now! Not when they begin to hear about me all over the solar system! Just a passing fancy! I suppose I might just as well have the machines bring her here now, to see just how completely helpless you two dopes are!"

Harwich and Paul Arnold were still pinioned to the floor by the automatons; but in the patrol pilot's slitted eyes glowed the subdued light of murder, futilely smoldering. The fat printer was absolutely master now of Clara, the boy, and himself. In his stupid, cruel, shallow vanity, cosmic power the deeper secrets of which he could never have understood, had driven Bayley to madness; to megalomania. That clanging and that red glow from near the distant hills showed the extent of his ambitions beyond question. The slave machines were not building that colossal fleet of space warships for nothing! Armed with weapons beyond human knowledge, such a fleet would sweep in aggressive fury to even the remotest world within the field of the sun's gravity!

But Harwich's feelings changed briefly to relief, when Clara Arnold was brought into the Tower room by another of those metal slaves. The automaton removed from her a flexible, transparent covering, of evidently airtight material, a protection against the rarity of the Ionian atmosphere, probably, for in being taken from the airlock of Bayley's rocket to the air-conditioned Tower here, she would otherwise have been exposed to suffocation.

The machine set the girl down gently. She looked scared, her blonde hair was awry, as though, maybe, she'd struggled with the robot; but otherwise she was still all right.

She looked about in wondering terror; for what she saw was still a complete mystery to her, just as it had been to her brother and Evan Harwich a little while ago. No one had told her anything yet.

"Paul—Evan!" she stammered "What is all this here? This pyramid, and Bayley? What's happened? Tell me, somebody!"

"Take it easy, Clara," Harwich responded, trying to sound reassuring. "Everything will be all right!" he ended a little unconvincingly, trying to shield the girl from grim truth.

"Everything's all right already, Clara," Bayley assured her mockingly. "I've got these two men of yours just where they can do the least harm! How would you like to see 'em squirm a little? I've got a special device for that purpose, something very refined and painful! And I've got just about everything else! In a month's time I could give you the planet Earth, to wear in a ring around your finger, if I happened to want to."

"What's he talking about, Evan?" the girl pleaded again, the shadow of fear in her face deepening. "It sounds sort of awful! Please tell me. Why are those flat monsters holding you and Paul to the floor?"

"I told you to take it easy, Clara," Harwich returned with a trace of sternness. "This maniac, Bayley, has got the upper hand now, but I said everything would be all right, didn't I?"

The patrol pilot was trying again to reassure the girl, with a show of truculent bravado this time. He hoped that truculence would make his words sound true, as though he had a trump card up his sleeve, or something.

"All right in the end, Harwich?" the fat printer chuckled wickedly. "Well, the end's pretty close. In another minute you'll be too tortured to do anything but scream. Right now I'm thinking and wishing. Look, the automatons are getting that agony tripod ready again!"

It was true. Metal tentacles were whipping about, adjusting the torture rods to touch Harwich's and Paul Arnold's skulls again.

Everything will be all right! That statement was a mocking memory to the patrol pilot now. An empty, rash challenge to the man whose petty ego yearned to control even the solar system.

Harwich had never felt so completely helpless in his life before, not even when he had been suffocating out there on the deserts of the Forbidden Moon. If he could only somehow knock Bayley out of that little, pillared structure that served as a receiver for telepathic orders to the machines; if only he could replace him there for a second, then everything might be very, very different! But Harwich was held helpless to the pavement of the tower room. His massive muscles were useless against machine might!

Direct argument—an attempt to make Bayley see the narrowness and lack of originality in his colossal ambitions—he knew was equally futile. Bayley was stubborn and shallow and greedy. Besides, he would never admit that he was wrong, even if he felt the truth of it!

So Harwich felt utterly checkmated on every side. The clanging out there, the building of the space fleet, mocked him. The rustle of wheels in that huge pyramid coordinator mocked him. All the Aladdin-like miracles of the Forbidden Moon mocked him, pointing out his impotence to do anything, now.

He even wondered savagely why that great coordinator mechanism, with all its terrific powers, didn't revolt against the dominance of the puny human being that mastered it. But, of course, it would have no desire to revolt. It had no desires of any kind, no capacity for happiness or misery, no consciousness even. It was no more alive, no more sentient, than an adding machine. Only infinitely more complex. It invented things and it directed lesser mechanisms only by the rolling of the wheels and the surge of energy inside it. And it responded to telepathic control of whomever was there to give it, just as a space ship might respond to whomever was at its throttle.

Still, there had to be some way out of this mess! Harwich knew it wasn't just Clara and Paul and himself that were in danger. It was everything he knew and respected. Freedom. Liberty. Unless he and his companions were able to do something, a Dark Age would come, surely. An age of machines, ruled by a madman.

The rod of the torture instrument was touching his skull. In just another moment the agony would begin. But what was Paul Arnold muttering beside him?

"Evan, those animals in the cages! We thought they looked like men didn't we? Here's something else: Maybe they are men, in a way! Men who went backward in evolution; lost their intelligence."

No one but Harwich could have heard the boy, for he spoke in a very low tone. But at once the patrol pilot understood; grasped a part of the Ionian riddle that he had missed before. Machines. No thinking or work to do. Indolence. And then?

At once Harwich saw a way, a slim possibility to avert cosmic catastrophe. He couldn't appeal to Bayley's reason, but maybe he could appeal to his fears. He had to try it, anyway.

Suddenly the patrol pilot's lips curled in derision and contempt. "Bayley," he said, "you're an utter damned fool! You think you'll extend your power all over the solar system. Well, maybe you will do that; but in the end you'll be destroyed! You give the orders—sure! But do you understand the thing in that pyramid? It was made to serve, as all machines are. The ancient Ionians had it pretty nice for themselves, yes. But did you ever wonder what happened to them? Where are they now? Do you know, Bayley?"

Harwich's final question was a dry whisper, like the voice of some ghost of ages past.

"Where are those ancient Ionians now, Bayley?" he repeated.

No man could have escaped awe there in that tremendous Tower room, where all the mysteries of the eons seemed to be congregated, many of them hidden and unknown and perhaps dangerous. George Bayley's eyes were suddenly very big. Quite evidently there were many things that he had not thought about. His gaze lingered momentarily on the great throbbing pyramid, inscrutable there in this huge dusky chamber.

"Stop trying to bluff me, you crazy idiot!" the fat printer stormed at last. "The Ionians are extinct, of course!"

Harwich managed to grin wolfishly. "If you believe that, Bayley, do you want to follow them into extinction?" he questioned. "Yes, they mastered science. They conquered even the problem of the thinning atmosphere and the loss of moisture and heat on their dying world. But after they turned their science over to the machines, something happened to them. Their numbers began to grow less, yes. They lost control of their empire, which must have included all the moons of—Jupiter. But they didn't completely die out, Bayley! Something happened to those Ionians that was far worse! Do you know what it was, Bayley? Do you want the same thing to happen to you?"

"I don't know what you're talking about!" the printer stammered furiously, fear of the unknown spreading over his plump face.

"No, those ancient people of the Forbidden Moon didn't become completely extinct," Harwich continued. "I believe you can see quite a few of them from the Tower room here. The walls are semi-transparent, and those cages outside aren't far away. They're full of Ionians. Sluglike, brainless monstrosities without even intelligence enough or will enough to wish any more!"

Harwich paused to let the facts sink into George Bayley's mind.

"That's them!" the patrol pilot continued. "It's an old theory that any race has to keep struggling, thinking, working; otherwise it goes backwards. By using their brains and muscles, Earthmen developed from apish ancestors, you know. But here the Ionians had everything done for them. So evolution was reversed. They lost their intelligence. And now, what are they? Stupid beasts, tended by machines that follow the original orders of long ago to take care of them. Worse than animals in a zoo."

Bayley's eyes were fairly popping, as he stared through the semi-transparent walls of the Tower room. Doubtless he could see those creatures in their air-conditioned habitations. Just helpless, squirming, incubator freaks!

"I wondered what they were—why they were here," Bayley stammered.

Harwich almost believed at first that he had won a point with the obese loan shark—scared him out of most of his wild ambitions. But then, gradually, he saw Bayley's expression grow a trifle less tense. It was just as Harwich had feared. The printer was beginning to realize that it must have taken countless generations to degenerate to their present sorry state. The same condition could not affect him personally. When Bayley saw this truth, he would be the same megalomaniac as before.

There was only that one slim chance left for Harwich. Bayley's attention was strongly diverted now. But in a few seconds more, he would be himself again.

Was the grip of the metal tentacles that held Harwich a little looser than before, now, because Bayley, the master of machines, had his mind so intensely on other things, and away from the thought of giving telepathic commands?

In a sudden, savage lunge, Harwich jerked free from the automaton that held him to the floor. His clothing was torn and his flesh scraped, but what did this matter? Everything depended on instant action. The patrol pilot leaped past Paul Arnold, and his sister, Clara, who had only watched and listened while he had talked with such grim truth to Bayley.

Already the flat, glittering robot was after Harwich, but he continued his surprise rush toward the roofed, pillared kiosk that was the receiver for telepathic orders.

His attack ended in a dying tackle. Bayley was drawing his heat pistol, but before he could fire it, Harwich's weight struck him. There, together, in the kiosk, they wrestled and fought. At last there was a chance for the patrol pilot to bring his massive muscles into play. He swung his heavy fists, and all the fury of weeks of hardship and misfortune were back of his blows. Bayley tottered away from under the kiosk, and for a second Harwich stood there free.

He was in the position of control at last; but Bayley had his pistol out and aimed, now. Clara was screaming as the fat man pressed the trigger.

It was too late for Harwich to marshal his thoughts properly. He was only able to will that the automaton behind him should cease attacking him. He could not call to his aid any of the great science of Io, in time.

With the speed of light, a slender pencil of intense heat waves from Bayley's pistol, struck his side and burned straight through his body. No bullet could have drilled a neater hole. Harwich's legs collapsed under him, and he lay writhing there within the kiosk.

A split second later the heat pistol in Bayley's hand spat again. Turning weakly, Harwich saw Clara crumple and go down. In another instant, Paul became the third victim.

"You're done, Harwich!" the fat printer was yelling triumphantly. "You're finished, all of you!"

But by now the patrol man's seething flood of hate had registered. He was within the telepathy kiosk; and if he had ever willed instant destruction for anyone, he willed it now, for Bayley. Under other circumstances he might not have felt so vengeful, but his ebbing pulses blazed with fury.

There was a click within that vast, slumberous pyramid, that loomed like a grim god in this shadowy place of enigmas. The automaton that had recently held Harwich captive, seemed to move like a maddened animal, created out of pure lightning. Its tentacles whipped around Bayley long before he could fire again. Harder than steel cable, the tendrils tightened, like the coils of a python.

There was a choked cry of terror and anguish, and then a sickening, crunching, squashing sound, as flesh and bone and blood oozed between those constricting metal loops.

It was almost the last thing that Evan Harwich saw. He was mortally wounded, a slender hole bored through his side.

Harwich's last delirium was a dream. A silly dream, maybe. Clara and he together. A little house. Fancifully he pictured its details. Maybe a mining concession somewhere here among the moons of Jupiter, too. An orderly life. Not all this hectic battling with unknown dangers any more. He was a little tired of adventure, a little tired of being space patrol pilot, too. He could resign.

Somewhere, Evan Harwich's fanciful thinking came to an end.

He awoke suddenly. Paul Arnold was shaking him.

"On your feet, you big lug!" the boy was yelling happily. "There's not a thing wrong with you, now! Clara and I have been awake for half an hour."

Harwich staggered erect, grumbling confusedly, his stiff, black hair awry. He'd been lying on a divan. The room around him was almost familiarly furnished, except for slightly fantastic details of decoration. The windows were wide, and beyond them there was a sort of yard, with freshly planted trees. Over the whole setup there was a fine crystal airdrome.

"What the heck! Where in the name of sense are we?" Harwich burst out in startled pleasure.

He looked first at Paul Arnold, and then at Clara, whose amber eyes were twinkling with secretive mischief. It was as though the two had some sort of joke up their sleeves.

Harwich glanced again out of the window. Beyond the airdome, glinting and new, was what looked like improved mining equipment. Cropping out of the ground was the grayish, shiny stuff of a rich ore lode. And there was a space ship, too; bright and slender and strange, but it looked plenty serviceable!

"Where are we, anyway?" Harwich demanded again, still completely in the dark. "Does either of you two know?"

"Still on Io, evidently!" Paul Arnold breezed with a taunting grin. "Same kind of hills and general character of country! When Bayley shot me, I passed out. I didn't know anything more until I woke up here a little while ago!"

"But this layout, Paul!" Harwich growled. "This house and this mining stuff! How come? You've got some kind of an answer in mind, I'm sure, by the way you look! I give up. Spill the gag!"

"Okay, Evan," said the boy. "I really do think I've got that part figured out! After Bayley shot you with the heat-pistol, you were lying in that telepathy kiosk in the Tower room. Consciously or unconsciously, you must have done some wishing there, before your brain blacked out."

Harwich gasped. So that was it! He'd wanted to be alive, though he had been mortally wounded. And so he was! His shirt was open. There was a neat round scar on his chest, left by the heat-ray burn, and evidence of careful supersurgery! The automatons of the Forbidden Moon had saved his life. Probably Clara's and Paul's lives, too. All while they were unconscious! The house, the garden, the mine!

"Our miracle hunt on the Forbidden Moon hasn't turned out so badly," Paul Arnold remarked. "But so far it's been a lot different from what Dad or you or I could have anticipated. This place looks like a nice family setup, Evan. Did you wish include anybody besides yourself?"

Harwich flushed, and looked sheepish. Clara, there, was definitely blushing, but she was smiling, too.

The ex patrol pilot managed a nervous grin. "I guess you got me there, Paul," he said. "Now, if it's all right with you, Clara, I don't know whether I have to say it or not, since it's a dead giveaway. But will you marry me?"

He got it out, feeling that it had been an awful job. But Clara smiled happily.

"Try and stop me, Evan," she laughed. "There has to be someone around to keep you from getting conceited. Just because you won out for us here on Io, doesn't mean that you won't need bossing yourself, once in a while!"

Paul Arnold winked, and left discreetly for other parts of the house.

Arm in arm Clara and Evan looked through a window that faced west. Something was flying there, high up in the sky. It glinted in the late afternoon sunlight. A lonely speck against the cold firmament, it seemed to hurry, bent on a last mission.

A few minutes later, from the east, there came a terrific concussion. The whole dark purple sky, above those sullen hills, was illuminated with a bluish-white glare for a second. Flying fragments soared far into space.

Clara clung tightly to Evan. "What was that?" she questioned fearfully.

Harwich grinned, but still there was a haunting shadow of sadness in his face. "I'm sure I know," he said. "That was the end of the science of the Forbidden Moon. The end of the force shield, apparatus, the end of those poor Ionians, and the end of the pyramid! The end of the whole thing. Suicide, you might call it. You see, back there in the telepathy kiosk, I wished that too, and the machines were made only to obey. I hope that when Earthmen, in the future, learn as much science as existed here on Io, they'll know how to use it, too. We're much too young a race yet, I guess."

Clara Arnold's awe softened after a moment. "Come on, Evan," she said. "Let's forget all about that for now. I want to show you the kitchen, here. It's ducky!..."