Benjamin Marlowe and his young assistant, Larry Powell, opened the door of the Marlowe laboratory, then stopped aghast at the sight which greeted their startled eyes.

There on the central floor-plate directly in the focus of the big atomic projector stood the slender figure of Joan Marlowe, old Benjamin Marlowe’s niece and Larry Powell’s fiancee.

The girl had apparently only been awaiting their return to the laboratory for around her gray laboratory smock was already fastened one of their Silver Belts, and a cord was already in place running from her wrist to the main switch of the projection mechanism.

Joan’s clear blue eyes sparkled with the thrill of high adventure as she swiftly raised a slender hand in a gesture of warning to the two men.

“Don’t try to stop me,” she warned quietly. “I can jerk the switch and be in Arret, before you’ve taken two steps. I’m going to Arret, anyway. I was only waiting for you to return to the laboratory so I’d be sure of having you here to bring me back to Earth again before I have time to get into any serious trouble over there.”

“But, Joan,” Benjamin Marlowe protested, “this is sheer madness! No one can possibly guess what terrible conditions you may confront in Arret. We’ve never dared to send a human being across the atomic barrier yet!”

“We’ve sent all kinds of animals across, though,” Joan retorted calmly, “and as long as we recalled them within the twelve-hour limit they always came back alive and unhurt. There’s no reason why a human being should not be able to make the round trip just as safely. Ever since our Silver Belts first came back with the weird plant and mineral fragments which proved that there really is such a place as Arret, I’ve been wild to see with my own eyes the incredible things that must exist there.”

Joan waved her hand in gay farewell. “Good-by, Uncle Ben and Larry! I know that you’ll drag me back just as quickly as you can possibly dash over to the recall switch, but I’ll at least have had a few precious seconds of sightseeing as Earth’s first human visitor to Arret!”

Larry Powell was already sprinting for the mechanism as Joan jerked the cord that ran to the switch, but he was barely half-way across the intervening space when the big atomic projector flared forth in a brilliant gush of roseate flame.

For a fraction of a second Joan’s slender figure was outlined in the very heart of the ruddy glow, then vanished completely. There was left only a short length of the switch cord to indicate that the girl had ever stood there.

Powell reached the mechanism and shut off the projector’s flame, then turned swiftly to the control-panel of the recall mechanism. As he closed the switch on this panel, three banks of tubes set in triangular form around the floor-plate upon which Joan had stood glowed a brilliant and blinding green.

Shielding his eyes from the glare with an upraised forearm, Powell began stepping a rheostat up to more and more power. In his anxiety, he increased the power far too quickly. There was a sudden gush of blue-white flame from the heart of the mechanism, together with the hissing crackle of fusing metal. The green light in the tubes promptly died.

Benjamin Marlowe was bending over the apparatus almost instantly. A moment later he raised a face that had suddenly gone white. There was terror in his eyes as he turned to his assistant.

“The entire second series of coils is burned out, Larry!” he gasped in consternation. “Joan is marooned over there in Arret--marooned in that grim unknown land as completely beyond our reach as though she were upon one of the moons of Mars!”

For a long moment the two men gazed at each other with horror-stricken faces, dazed and shaken. Then they quickly drew themselves together again and set about the herculean task of making the necessary repairs to the damaged mechanism in time to rescue Joan before the twelve-hour limit should doom the girl to forever remain an exile in that land of alien mystery beyond the atomic barrier.

Their previous experiments with animals had proved that no living creature from Earth could be brought back after it had been in Arret over twelve hours. After that time the change in the atoms constituting living tissues apparently became permanently Arretian, for the Silver Belts returned without any trace of their original wearers.

The necessary repairs to the damaged coils were of such an exacting and intricate nature that any great speed was impossible. Hours passed while the two men bent to their work with grim concentration. Neither of them dared think too much of what nameless dangers might be confronting Joan during those weary hours. Their actual knowledge of Arret was so pitiably slight.

Some months ago, while they were experimenting upon apparatus for reversing the electrical charges of an atom’s electrons and protons, they had first stumbled upon the incredible fact that such a place as Arret really existed. They found that it was another world occupying the same position in space as Earth, with the fundamental difference in the two interwoven planes of existence lying in the electrical make-up of the atoms that constituted matter in each plane.

On Earth all atoms are composed of small heavy protons that are always positive in charge, and larger lighter electrons that are always negative. In Arret the protons were negative, and the electrons positive. The result was two worlds occupying the same space at the same time, yet with matter so essentially and completely different that each world was intangible to the other. They had named the unseen world Arret, the reverse of Terra.

Finding it impossible to work directly upon most forms of matter, the experimenters had finally evolved a silver alloy that served as a medium both for sending objects into Arret and then bringing them back to Earth. By focussing the flame of the projection apparatus upon a Silver Belt of this alloy, the electrical charges of the Belt’s atoms were reversed, automatically causing the Belt to vanish from Earth and materialize in Arret. At the same time the atoms of any object within the Belt’s immediate radius were similarly transformed, and that object was taken into Arret with the Belt.

The recall mechanism functioned by broadcasting a power wave that again reversed the atomic charge of the Belt and its contained object back to that of Earth. At the same time the recall wave exerted an attractive force that drew the atoms back to a central point in the laboratory, where they were re-materialized upon the same floor-plate from which they had originally been sent.

The twelve-hour time limit was half up when Benjamin Marlowe and Larry Powell finally straightened up wearily from their work over the recall mechanism, their repairs completed. It had been one o’clock in the afternoon when Joan Marlowe vanished from Earth in the roseate flare of the projector. It was now nearly seven o’clock.

With nerves tense from anxiety, the two men crossed over to the control-panel of the recall apparatus. This time they donned goggles of dark glass to shield their eyes from the blinding green glare. Marlowe threw the main switch, and the banked tubes came to life in a flood of vivid emerald light.

Marlowe began stepping the rheostat up gradually to more power, advancing it with cautious slowness to avoid any chance of a repetition of the previous accident. The green radiance streaming from the tubes in every direction began to throb with an electric force that the two men could feel pulsing through their own bodies.

There was a click as the rheostat struck the last notch. The green radiance was now a searing flame that half-blinded them even through the thick dark glass of their protective goggles, while the vibrant force of the green rays was sweeping through their bodies with a tingling shock that nearly took their breath away.

Tensely the two men stared at the metal floor-plate in the center of the area bounded by the flaming green tubes. Just over the plate the green radiance seemed to be thickening and swirling oddly. The swirling eddy became a small dense cloud of darker green light. Then abruptly, like the fade-in on a moving picture screen, from the cloud over the plate the misty outlines of an object swiftly cleared and solidified into a bizarre something at whose unfamiliar aspect both Marlowe and Powell gasped in amazement.

Marlowe snapped the switch off, and the green radiance vanished. Stripping the dark goggles from their eyes, the two men hurried over for a closer view of the thing that rested quiescent and apparently lifeless there on the metal floor-plate.

It was shaped like a huge egg, a little over a yard long, and was apparently composed of a solid lump of some unknown crystalline substance that closely resembled very clear, pale amber. Embedded in the heart of the strange egg were clearly visible objects which caused Marlowe and Powell to gasp in mingled horror and amazement.

Chief among the things imprisoned in that amber shroud was the Silver Belt that Joan had worn, but the Belt was now looped over the bony shoulder of a skeleton that by no possible stretch of the imagination could ever have been that of a creature of this Earth.

The skeleton was still perfectly articulated, and gleamed through the crystalline amber as though its bony surfaces were encrusted with diamond dust. The bones were apparently those of a creature that in life had been half dwarf-ape and half giant rat.

The beast had stood a little under a yard in height. The legs were short, powerful, and bowed. The long arms ended in claw-like travesties of hands. The skull was relatively small, with a sharply sloping forehead and projecting squirrel-like teeth that were markedly rodent.

Around the skeleton’s neck there was a wide band of some strange gray metal, with its smooth outer surface roughly scratched in characters that resembled primitive hieroglyphics.

Marlowe’s face was white with grief as he turned to Powell. “Joan must be dead, Larry,” he said sadly. “Otherwise, she would surely never have allowed her Silver Belt to pass into the possession of--this! She knew that the Belt represented her only hope of ever being brought back to this world.”

For a moment Powell stared intently into the heart of the crystalline egg without answering. Then suddenly he straightened up with marked excitement upon his face.

“There’s a small sheet of paper entwined in the coils of that Belt!” he exclaimed. “It may be a message from Joan!”

Swiftly the two men lifted the amber egg up to the top of a workbench. Powell took a small hammer to test the hardness of the strange translucent substance.

He struck it a sharp rap, then recoiled in surprise at the effect of his blow, for the entire egg instantly shattered with a tinkling crash like the bursting of a huge glass bubble. So complete was the disintegration of the egg and the skeleton within it that all that remained of either was a heap of diamond and amber dust. The only things left intact were the Silver Belt and the metal collar.

Powell snatched up the Belt and extracted the small piece of paper that had been firmly tucked into its coils. Hurriedly written in pencil upon the paper was a message in a handwriting familiar to both Powell and Marlowe:

Help! I am held prisoner in the Cave of Blue Flames!

--Joan.

“Larry, Joan must still be alive over there in Arret!” There was new hope in Benjamin Marlowe’s voice.

“Yes, alive and held captive by whatever monstrosities may inhabit that unknown plane,” Powell agreed grimly. “There’s only one way in which we can possibly rescue her now. That is for you to send me into Arret with a reserve Belt for Joan. I’ll be ready to start as soon as I get a couple of automatic pistols that I have up in my room. It’s a sure thing that I’ll need them over there in Arret.”

Five minutes later Powell stood ready and waiting upon the floor-plate in the focus of the big atomic projector, with the central lens of the apparatus levelled down upon him like a huge searchlight. Around Powell’s waist were strapped two Silver Belts, and a cartridge belt with a holstered .45-calibre automatic on either side. His wrist-watch was synchronized to the second with Benjamin Marlowe’s watch.

“Joan’s twelve-hour time limit in Arret will expire at one o’clock tomorrow morning.” Powell reminded Marlowe. “That gives me nearly six hours in which to find her and equip her with a Silver Belt. You will broadcast the recall wave at exactly one o’clock. If I haven’t succeeded in finding Joan by then, I’ll discard my own Belt and stay on over there in Arret with her ... I’m ready to start now, whenever you are.”

Benjamin Marlowe raised his hand to the switch in the projector’s control panel. “Good-by, Larry,”--the old man’s voice shook a trifle in spite of himself--”and may God be with you!” He closed the switch.

A great burst of roseate flame leaped toward Powell from the projector. The laboratory was instantly blotted out in a swirling chaos of ruddy radiance that swept him up and away like a chip upon a tidal wave. There was a long moment during which he seemed to hurtle helplessly through a universe of swirling tinted mists, while great electric waves tingled with exquisite poignancy through every atom of his body.

Then the mists suddenly cleared like the tearing away of a mighty curtain, and with startling abruptness Powell found himself again in a solid world of material things. For a moment as he gazed dazedly about him he thought that the roseate glow of the projector must still be playing tricks with his eyesight, for the landscape around him was completely and incredibly red!

He soon realized that the monochrome of scarlet was a natural aspect of things in Arret. The weird vegetation all around him was of a uniform glossy red. The sandy soil under his feet was dull brick-red. High in the reddish-saffron sky overhead there blazed a lurid orb of blood-red hue, the intense heat of its ruddy radiance giving the still dry air a nearly tropical temperature. From this orb’s position in the sky and its size, Powell was forced to conclude that it must be the Arretian equivalent of Earth’s moon.

For a moment he stood motionless as he peered cautiously around him, trying to decide what should be his first step in this scarlet world that was so utterly alien in every way to his own. On every side the landscape stretched monotonously away from him in low rolling dunes like the frozen ground swell of a crimson sea--dunes covered with vegetation of a kind never seen upon Earth.

Not a leaf existed in all that weird flora. Instead of leaves or twigs the constituent units of bushes and grasses consisted of globules, glossy spheres of scarlet that ranged in size from pinheads to the bulk of large pumpkins. The branches of the vegetation were formed from strings of the globules set edge to edge and tapering in size like graduated beads strung upon wire, dwindling in bulk until the tips of the branches were as fragile as the fronds of maidenhair fern. The bulk of the shrubbery was head-high, and so dense that Powell could see for only a couple of yards into the thicket in any direction.

The stillness around Powell was complete. Not even a globular twig stirred in the hot dry air. Powell decided to head for the crest of one of the low dunes some fifty feet away. From its top he might be able to sight something that would give a clue to the location of the “Cave of Blue Flames” of which Joan had written.

He arrived at the foot of the dune’s slope without incident. But there he came to an abrupt halt as the silence was suddenly shattered by a strange sound from the shrubbery-covered crest just above him. It was a musical, tinkling crash, oddly suggestive of a handful of thin glass plates shattering upon a stone floor. A second later there came the agonized scream of some creature in its death throes.

The tinkling, crashing sound promptly swelled to a steady pulsing song like that of a brittle river of crystalline glass surging and breaking over granite boulders. There was an eery beauty in that tinkling burst of melody, yet with the beauty there was an intangible suggestion of horror that made Powell’s flesh creep.

The crystalline song swelled to a crescendo climax. Then there came another sound, a single resonant note like that given when a string of a bass viol is violently plucked--and the tinkling melody abruptly died. Immediately following the resonant twang some object was ejected from the midst of the thicket on the dune’s crest, and came rolling and bounding down the gentle slope toward Powell.

It finally came to rest against the base of a bush almost at his feet. He whistled softly in surprise as he saw the nature of the thing. It was another of the yard-long egg-shaped crystals of translucent amber like the one that had been materialized in Benjamin Marlowe’s laboratory. Imprisoned in the clear depths of this amber egg was the sparkling, diamond-encrusted skeleton of what had apparently been a small quadruped about the size of a fox.

Powell’s eyes narrowed in speculation as he realized that he had before him the first slight clue as to what might have happened to Joan. Her Silver Belt had been enclosed in one of those amber, crystalline eggs. Apparently her capture had been in some way connected with that sinister, unseen Tinkling Death.

Powell began cautiously working his way up the slope of the dune, with an automatic pistol ready for use in his right hand. Silence reigned unbroken now in the thicket on the crest, but with each upward step that he took there came with constantly increasing force a feeling of some vast, alien intelligence lurking up there, watching and waiting.

Nearer and nearer the crest he worked his wary way, until he was so close that he fancied he could see the vague outline of some monstrous silvery bulk looming there in the heart of the red thicket. He took another cautious step forward--and then his careful stalking was sharply interrupted.

Without a second’s warning there came the roaring rush of great wings beating the air just above him. Powell tried to dive for cover, but he was too late. A slender snaky tentacle came lashing down and struck his shoulder with a force that sent him sprawling forward upon his face. Before he could rise, two of the tentacles twined around him, and he was jerked up into the air like a wood-grub captured by a husky robin.

Again the great wings above him threshed the air in tremendous power, as the unseen monster started away with its prey. Then the tentacles from which he was dangling shifted their grip slightly, turning Powell’s body in the air so that he could look up and get his first glimpse of the thing that had captured him. He shuddered at what he saw. The creature was a hideous combination of octopus and giant bat.

Naked wings of membrane spanned twenty feet from tip to tip. There was a pursy sac-like body, ending in a head with staring, lidless eyes and a great black beak that looked strong enough to shear sheet steel. From the body descended half a dozen long writhing tentacles.

Powell’s one hundred and eighty pounds made a weight that was apparently a burden for even this flying monster. It flew jerkily along, scarcely a dozen feet from the ground, and there was laborious effort obvious in every movement of its flapping wings. Powell decided to make a prompt break for escape before the octopus-bat succeeded in fighting its way any higher. His left arm was still pinioned to his body by one of the constricting tentacles, but his right hand, with the automatic in it, was free.

He swung the weapon’s muzzle into line with the hideous face above him, then sent a stream of lead crashing upward into the creature’s head. The bullet struck squarely home. The tentacles tightened convulsively with a force that almost cracked Powell’s ribs. Then in another paroxysm of agony the tentacles flung him free.

The impetus of his fall sent him rolling for a dozen feet. Unhurt, save for minor scratches and bruises, he scrambled to his feet just in time to see the mortally wounded octopus-bat come crashing down in the red vegetation some thirty yards away. For a few minutes there was audible a convulsive threshing; and then there was silence.

Powell refilled the automatic’s clip, then looked about, trying to regain his bearings. He wanted to return to the thicket of the Tinkling Death, but the octopus-bat had carried him hundreds of yards from there and he was now uncertain even of the direction in which the thicket was.

As he paused in indecision, there came to Powell’s ears a new sound that promptly drove all thought of the Tinkling Death from his mind.

The sound of his gun against the octopus-bat had apparently attracted new and unseen assailants--and their number was legion. Swiftly closing in upon him from every side there came the rustle and whisper of countless thousands of unseen foes advancing through the dense red thickets.

Completely hemmed in as he was, flight was out of the question. He sought the center of a small clearing, some ten feet in diameter, in order to gain at least a moment’s sight of his adversaries before they swarmed in upon him. With an automatic in each hand, he waited tense and ready.

The encircling rush came swiftly nearer, until Powell was suddenly aware that the unseen horde had arrived. The thicket bordering his tiny clearing was literally alive with yard-high furry bodies of creatures that dodged about too swiftly in the cover of the red bushes for him to get a clear view of any of them. There was a constant babel of snarling, chattering sound as the things called back and forth to each other.

Then the chattering stopped abruptly, as though at the command of some unseen leader. The next moment one of the creatures stepped boldly out into full view in the clearing. Powell’s scalp crinkled in disgust as he realized the nature of the thing confronting him.

It was literally a rat-man. Its upright posture upon two powerful, bowed hind legs was that of a man, but its human-like points were overshadowed by a dozen indelible marks of the beast. A coat of short, dirty gray fur covered the creature from head to foot. Its hands and feet were claw-like travesties of human members. Its pointed, chinless face with its projecting teeth and glittering little beady eyes was that of a giant rodent.

The beast in the clearing was apparently a leader of some sort, for around his throat was a wide collar of gray metal, with its flat surface marked in rudely scratched hieroglyphics. Powell’s heart leaped as he noted the collar. In this creature before him he had his second clue to the whereabouts of Joan Marlowe.

Not only was the collar practically identical to the one worn by the skeleton that had been materialized in the egg back in the laboratory, but the skeleton itself was obviously that of one of the rat-men. Could it be this grotesque horde of human-like rodents that was holding Joan captive in the Cave of Blue Flames?

Powell tried desperately to think of some way of communicating with the gray-collared leader. Then the beast shrilled a command that brought hundreds of the beasts swarming into the clearing from every side, and in the face of the menace of their countless glittering eyes and bared fangs Powell abandoned all thought of attempting to parley with the beasts.

There was another shrill command from the leader, and the horde closed in. Both of Powell’s guns flamed in a crashing leaden hail that swept the close-packed ranks of furry bodies with murderous effect. But he was doomed by sheer weight of numbers.

The rat-men directly in front of the blazing pistols wavered momentarily, but the press of the hundreds behind them swept them inexorably forward. Powell emptied both guns in a last vain effort. Then he was swept from his feet, and the horde surged over him.

Blinded and smothered by the dozens of furry bodies that swarmed over him, he had hardly a chance to even try to fight back. His cartridge-belt and guns, his Silver Belts and his wrist-watch were stripped from him by the dozens of claw-like hands that searched his body. Other claw-hands jerked his arms behind his back and lashed them firmly together with rope.

A blanketing sheet of some heavy fabric was crammed over his head and tied in place so tightly that he was completely blindfolded and half-suffocated. A noose was knotted around his neck. A suggestive jerk of this noose brought Powell lurching to his feet; there was another commanding jerk, and he obediently started walking.

The march that followed soon became torture for the captive. Blindfolded as he was, and having only the occasional jerks of rope to guide his footsteps, he stumbled and fell repeatedly, until his aching body seemed one solid mass of bruises.

As nearly as he could judge, the horde had conducted him nearly two miles when the path abruptly sloped downward. A moment later the sudden coolness of the air and the echoes about him told him that they had entered an underground passage of some kind. After traversing this passage for several yards they emerged into what was apparently a large open area, for he could hear the excited chattering and squealing of countless thousands of rat-men on every side of him.

He was dragged forward a dozen steps more, then brought to a halt. The blindfolding fabric was roughly stripped from his head. For a moment he blinked dazedly, half-blinded by a glare of blue light that flooded the place.

He was standing in a vast cavern. From dozens of fissures high in the rock walls streamed flickering sheets of blue flame which both warmed and lighted the place. There was a weird tingling glow in the air that suggested that the strange blue fires might be electrical in their origin.

Powell looked eagerly around for Joan, but he could see no trace of her. The only other living beings in the big cavern were the swarming thousands of the rat-people. The brutes were apparently too low in the evolutionary scale to have any but the most primitive form of tribal organization.

Sitting on a rude rock throne just in front of Powell was a grotesquely fat, mangy-furred old rat-man who was obviously the king of the horde. Some thirty or forty rat-men, larger and stronger than their fellows, wore the gray-metal collars that apparently marked them as minor leaders.

The great bulk of the horde, numbering far into the thousands, swarmed in the cavern in one vast animal pack, sleeping, feeding, snarling, fighting. As Powell was halted before the king’s throne, most of them abandoned their other pursuits to come surging around the captive in a jostling, curious mob.

The metal-collared leader of the pack that had captured Powell presented the rat-king with the captive’s gun-belt and two Silver Belts, accompanying the gifts with a squealing oration that was apparently a recital of the capture. The old monarch took the trophies with delight.

The two Silver Belts were promptly draped over his own furry shoulders by the king--seemingly following the same primitive love for adornment that inspires an African savage to ornament his person with any new and glittering object he happens to acquire. The rat-king then graciously draped the cartridge-belt and holstered automatics around the shoulders of the metal-collared leader who had captured Powell.

The king turned his attention back to his prisoner. He studied the captive curiously for a moment or two, then squealed a brief command. A score of the rat-men promptly closed in upon Powell, and began herding him toward a far back corner of the big cavern.

Stopping a few yards away from the edge of what seemed to be a wide deep pit in the rock floor, the guard stripped Powell’s bonds from him. Powell made no move to take advantage of his freedom, realizing that the swarming thousands of rodents in the cave made escape out of the question for the moment. He allowed himself to be docilely herded on to the edge of the pit.

And the next moment he exclaimed aloud in delighted surprise as he gazed down at the floor of the pit ten feet beneath him. There, sitting on a low heap of stones on the pit’s sandy floor, white-faced and weary but apparently unhurt, was Joan Marlowe.

The girl’s face brightened in relief as she looked up and recognized him.

“Larry! Oh, thank God you’ve come!”

The leader of the guards motioned for Powell to jump down into the pit. He needed no urging. A moment later he landed lightly on the sandy floor of the pit, and Joan was in his arms.

The rat-men left a dozen of their number scattered as sentries around the edge of the pit. The rest of them returned to the main horde, leaving the prisoners to their own devices.

“I knew that you’d come, Larry, as soon as you got my note,” Joan exclaimed happily. “But how did you ever succeed in finding this Cave of Blue Flame?”

“I didn’t find it myself,” Powell admitted. “I was captured like a boob and dragged here.” He told Joan of his mishaps since arriving in Arret.

The girl nodded when he had finished. “Much the same happened to me, Larry, only the red moon wasn’t shining then. The only light was from what looked like the dim ghost of a big yellow sun. I materialized in Arret almost in the middle of a scouting group of rat-men. They took me captive immediately. When several minutes passed without you and Uncle Benjamin broadcasting the recall wave for me, I knew that something terrible must have happened back in the laboratory, and that I might be marooned in Arret for hours.