On impulse, he climbed the outside stair and rapped on the door of the garage apartment. When the deep slow voice of Aurora’s tenant told him to enter, he found the old vagabond seated on a tall stool, bent over his intricate equipment assembled on the kitchen table.

To his relief, the shabby little apartment had not been changed. The glossy walls of his own new room were something which burned at night with a pale golden fire until the humanoid stopped it, and the new floor was something warm and yielding, which felt almost alive; but these little rooms had the same cracked and water-stained plaster, the same cheap fluorescent light-fixtures, the same worn carpets over splintered floors.

“How do you keep them out?” he asked, wistfully. “Those damned mechanicals?”

The stooped and gaunt old man rose stiffly to move a pair of pliers and some odds and ends of sheet metal off a crippled chair, and motioned graciously for him to be seated.

“I have a certain immunity,” Sledge told him gravely. “The place where I live they cannot enter, unless I ask them. That is an amendment to the Prime Directive. They can neither help nor hinder me, unless I request it—and I won’t do that.”

Careful of the chair’s uncertain balance, Underhill sat for a moment, staring. The old man’s hoarse, vehement voice was as strange as his words. He had a gray, shocking pallor. His cheeks and sockets seemed alarmingly hollowed.

“Have you been ill, Mr. Sledge?”

“No worse than usual. Just very busy.” With a haggard smile, he nodded at the floor. Underhill saw a tray where he had set it aside, bread drying up and a covered dish grown cold. “I was going to eat it later,” he rumbled apologetically. “Your wife has been very kind to bring me food, but I’m afraid I’ve been too much absorbed in my work.”

His emaciated arm gestured at the table. The little device there had grown. Small machinings of precious white metal and lustrous plastic had been assembled with neatly soldered busbars into something which showed purpose and design.

A long palladium needle was hung on jeweled pivots, equipped like a telescope with exquisitely graduated circles and vernier scales and driven like a telescope with a tiny motor. A small concave palladium mirror, at the base of it, faced a similar mirror mounted on something not quite like a rotary converter. Thick silver busbars connected that to a plastic box with knobs and dials on top, and also to a foot-thick sphere of gray lead.

The old man’s preoccupied reserve did not encourage questions, but Underhill, remembering that sleek black shape inside the new windows of his house, felt queerly reluctant to leave this haven from the humanoids.

“What is your work?” he ventured.

Old Sledge looked at him sharply, with dark feverish eyes, and finally said: “My last research project. I am attempting to measure the constant of the rhodomagnetic quanta.”

His hoarse tired voice had a dull finality, as if to dismiss the matter and Underhill himself. But, haunted with a terror of the black shining slave that had become the master of his house, Underhill refused to be.

“What is this certain immunity?”

Sitting gaunt and bent on the tall stool, staring moodily at the long bright needle and the lead sphere, the old man didn’t answer.

“Those damned mechanicals!” Underhill burst out nervously. “They’ve smashed my business and moved into my home.” He searched the old man’s dark, seamed face. “Tell me—you must know more about them—isn’t there any way to get rid of them?”

After a half a minute, the old man’s brooding eyes left the lead ball, and the gaunt shaggy head nodded wearily.

“That’s what I am trying to do.”

“Can I help you?” Underhill trembled to a sudden eager hope. “I’ll do anything.”

“Perhaps you can.” The sunken eyes watched him thoughtfully, with some strange fever in them. “If you can do such work.”

“I had engineering training,” Underhill reminded him. “I’ve a workshop in the basement. There’s a model I built.” He pointed at the trim little hull hung over the mantel in the tiny living room. “I’ll do anything I can.”

Even as he spoke, however, the spark of hope was drowned in a sudden wave of overwhelming doubt. Why should he believe this old rogue, when he knew Aurora’s taste in tenants? He ought to remember the game he used to play, and start counting up the score of lies. He stood up from the crippled chair, staring cynically at the patched old vagabond and his fantastic toy.

“What’s the use?” His voice turned suddenly harsh. “You had me going there. I’d do anything to stop them, really. But what makes you think you can do anything?”

The haggard old man regarded him thoughtfully.

“I should be able to stop them,” Sledge said softly. “Because, you see, I’m the unfortunate fool who started them. I really intended them to serve and obey, and to guard men from harm. Yes, the Prime Directive was my own idea. I didn’t know what it would lead to.”

V

Dusk crept slowly into the shabby little rooms. Darkness gathered in the unswept corners and thickened on the floor. The toy-like machines on the kitchen table grew vague and strange, until the last light made a lingering glow on the white palladium needle.

Outside, the town seemed queerly hushed. Just across the alley, the humanoids were building a new house, quite silently. They never spoke to one another, for each knew all that any of them did. The strange materials they used went together without any noise of hammer or saw. Small blind things, moving surely in the growing dark, they seemed as soundless as shadows.

Sitting on the high stool, bowed and tired and old, Sledge told his story. Listening, Underhill sat down again, careful of the broken chair. He watched the hands of Sledge, gnarled and corded and darkly burned, powerful mice but shrunken and trembling now, restless in the dark.

“Better keep this to yourself. I’ll tell you how they started, so you will understand what we have to do. But you must not mention it outside these rooms—because the humanoids have very efficient ways of eradicating unhappy memories, or purposes that threaten their discharge of the Prime Directive.”

“They’re very efficient,” Underhill bitterly agreed.

“That’s all the trouble,” the old man said. “I tried to build a perfect machine. I was altogether too successful. This is how it happened.”

A gaunt haggard man, sitting stooped and tired in the growing dark, he told his story.

“Sixty years ago, on the arid southern continent of Wing IV, I was an instructor of atomic theory in a small technological college. A bachelor. An idealist. Rather ignorant, I’m afraid, of life and politics and war—of nearly everything, I suppose, except atomic theory.”

His furrowed face made a brief sad smile in the dusk.

“I had too much faith in facts, I suppose, and too little in men. I mistrusted emotion, because I had no time for anything but science. I remember being swept along with a fad for general semantics. I wanted to apply the scientific method to every situation, to reduce all experience to formula. I’m afraid I was pretty impatient with human ignorance and error. I thought that science alone could make the perfect world.”

He sat silent for a moment, staring out at the black silent things that flitted shadow-like about the new palace that was rising as swiftly as a dream, across the alley.

“There was a girl.” His great tired shoulders made a sad little shrug. “If things had been a little different, we might have married, and lived out our lives in that quiet little college town, and perhaps reared a child or two. There would have been no humanoids.”

He sighed, in the cool creeping dusk.

“I was finishing my thesis on the separation of the palladium isotopes—a petty little project, but I should have been content with that. She was a biologist, but she was planning to retire when we married. I think we should have been two very happy people, quite ordinary, altogether harmless.

“But then there was a war—wars had been too frequent on the worlds of Wing, ever since they were colonized. I survived it in a secret underground laboratory, designing military mechanicals. But she volunteered to join a military research project in biotoxins. An accident let a few molecules of a new virus escape into the air, and everybody on the project died unpleasantly.

“I was left with my science. That, and a bitterness that was hard to forget. The war over, I went back to the little college with a military research grant. The project was pure science—a theoretical investigation of the nuclear binding forces, then misunderstood. I wasn’t expected to produce an actual weapon, and I didn’t recognize the weapon when I found it.

“It was only a few pages of rather difficult mathematics. A novel theory of atomic structure, involving a new expression for one component of the binding forces. The tensors seemed to be a harmless abstraction. I saw no way to test the theory or manipulate the predicated force. The military authorities cleared my paper for publication in a little technical review put out by the college.

“The next year, I made an appalling discovery—I found the meaning of those tensors. The elements of the rhodium triad turned out to be an unexpected key to the manipulation of that theoretical force. Unfortunately, my paper had been reprinted abroad. Several other men must have made the same unfortunate discovery, at about the same time I did.

“The war, which resulted in less than a year, was probably started-by a laboratory accident. Men failed to anticipate the capacity of tuned rhodomagnetic radiations to unstabilize the heavy atoms. A deposit of heavy ores was detonated, no doubt by sheer mischance. The blast obliterated the incautious experimenter. Its cause was misunderstood.

“The surviving military forces of that nation retaliated against their supposed attackers, with their rhodomagnetic beams that made the old-fashioned bombs seem pretty harmless. A beam carrying only a few watts of power could fission the heavy metals in distant electrical instruments, the silver coins that men carried in their pockets, the gold fillings in their teeth, or even the iodine in their thyroid glands. If that was not enough, slightly more powerful beams could set off heavy ores, beneath them.

“Every continent of Wing IV was plowed with new chasms vaster than the ocean deeps, and piled up with new volcanic mountains. The atmosphere was poisoned with radioactive dust and gases. Rain fell thick with deadly mud. Most life was obliterated, even in the shelters.

“Bodily, I was again unhurt. Once more, I had been imprisoned in an underground site, this time designing new types of military mechanicals to be powered and controlled by rhodomagnetic beams—for war had become far too swift and deadly to be fought by human soldiers. The site was located in an area of light sedimentary rocks which were not easily detonated, and the tunnels were shielded against the fissioning frequencies.

“Mentally, however, I must have emerged almost insane. My own discovery had laid the planet in ruins. That load of guilt was pretty heavy for any man to carry; it corroded my last faith in the goodness and integrity of man.