I deliberately dug my heels into the concrete floor of the corridor of the Pentagon. The steel plates on the heels of my black uniform boots heralded my approach with sharp anvil sounds as I marched confidently toward the unmarked door five hundred feet ahead.

What was that expression used by Earth people of the 20th century? I shifted back through my training, shuffled through the facts about Earth's past history with which I had been indoctrinated, searching for the word. Assassin! That was it. But the term fell short. It lacked in magnitude. There was a difference in the murder of one person and the assassination of the occupants of an entire planet!

One foot in front of the other, I paced off the distance toward the end of the hallway, carefully duplicating the strut which was a trademark of the Earth Council's Security Police. I'd practiced the peculiar, jolting method of walking a thousand times, but I began to feel the effects of Earth's heavier gravity before I had covered half the distance. It had been impossible to simulate the difference in gravity in my training.

The two guards standing outside the door alertly watched my approach. When I was still four paces away, one of them ordered me to stop. They ignored as though they were not there the gold stars prominently displayed on the shoulders of my tunic.

The guard on the left said, "Your ID card, sir."

The guards were well trained. They would not hesitate to shoot if I made the slightest slip.

I handed the card to him and watched as he held it up to a visi-scanner in the wall. The scanner glowed into life and purred softly, rapidly checking the invisible identification codings on the card against the ID component of Earth's Master Machine. Then it dulled and was silent. The strident alarm siren over the scanner remained inactive. The ID card was returned to me and the guards snapped smartly to attention as I went on into the room beyond the door.

I had passed the first test.

The reception room was small. Thick carpeting deadened the clump of my heels as I marched toward the chromed desk guarding a second unmarked door. A flawlessly proportioned redhead sat behind the desk. Her eyes and face showed no expression when I stopped in front of her. Her tight-fitting uniform was black and bore the gold trim of the Security Police.

Constricting my throat, I let the words snap out crisply, as I had been trained.

"General Spicer," I said, "commanding general of the Security Police, reporting to the Secretary of Defense. As requested."

I waited.

Her eyes, still showing no outward expression, ran over me rapidly. Then she thumbed a button on the desk and a screen, recessed into the chromed surface, glowed into life.

Almost immediately, a full-face reproduction of the features of General Spicer appeared on the screen in color. She checked the image against my face, her eyes flickering to the tiny scar under my left eye and to the old blaster burn across my right ear. When the image changed to a profile view, I turned my head to give her the same angle.

She nodded, pressing the button on her desk which darkened the screen.

She said, "You're early. Your appointment with Secretary Bartlett is—"

"For 1300 hours," I filled in automatically, when she hesitated in one last routine test. "I was in the building on another matter, however, and came here after I had finished my other business."

"Yes, of course," she said. "Please take a seat. Senator Chambers is ahead of you, but his business will not take long."

I fought back the sudden impulse to pivot and stare in the direction her eyes were indicating. Senator Carl Chambers. My briefing on him had been lengthy. For 60 Earth years, he had headed the un-Earth Activities Committee. As General Spicer, I was supposed to have a nodding acquaintance with him, but no more than that. During the years, our rivalry had become legend. His unanticipated presence in the waiting room could prove disastrous. Chambers would not be fooled easily.

Turning slowly, I nodded stiffly and curtly in Chambers' direction and then selected a chair across the room from him.

The senator's head merged directly into the shoulders of his grossly rotund body. Small, round eyes stared unblinkingly at me from the red pudginess of his face. They hesitated on the black swagger stick which I held loosely in my right hand, moved on, and then returned to it. The invisible scars, made by the electro-surgical knives in re-designing my body, began to tense slowly. I shifted the swagger stick in my hand.

Then the redheaded secretary stood up. She said, "Secretary Bartlett will see you now. Senator."

For a fraction of a time, I thought Senator Chambers had not heard her. His expressionless eyes were still on me. Then, with a grunt, he lifted himself to his feet and disappeared through the door behind her. A tiny clicking noise indicated that it locked automatically.

I shifted my gaze and saw that the secretary was looking at me intently. It was impossible to guess at what might be going on behind those eyes. The tension began to build inside me again, but I kept my own eyes as expressionless as hers.

The girl picked up a folded piece of paper out of a receptacle on her desk and brought it over to me.

She said, "While you're waiting, General, you might like to read the latest facsimile. Or have you already seen it?"

I shook my head. "I saw the 1100 fac-report, but I missed this one."

She handed it to me and returned to her desk. There was just the slightest suggestion of a rolling movement in her walk, not at all unpleasant.

When I looked down at the facsimile sheet, the headline screamed silently up at me. I swiveled my eyes over at the secretary, but she was working her recordo-writer, her fingers moving rapidly, mechanically.

The headline read: ALIEN INVADER DISCOVERED! The story that followed reported that two Security Police guards had intercepted someone who looked like and was dressed like an Earthman, trying to enter the Senate at 1109 hours that morning. A discrepancy had been discovered during the routine ID card check and the imposter had tried to escape. The guards had opened fire at close range, scoring two direct hits.

While the account was obviously censored, it intimated that a full report to be released later by Security Police Headquarters would be almost unbelievable. It hinted that the hideous mess revealed when the guards' weapons had ripped through the surprisingly soft body armor of the impostor positively confirmed the fact that the individual was an enemy alien.

Before I could read any further, there was a muted tone from the direction of the desk. The secretary acknowledged the signal, spoke several words which I couldn't hear, then looked at me.

She said, "You may go in now, General Spicer."

I placed the facsimile sheet on her desk and waited while she activated the circuit, which would release the catch on her side of the door.

Who had it been? There had been four of us. Volunteers. We had been selected, briefed and trained separately. We had been housed separately during the mental and physical tortures of the surgical and the psych labs. The ship which had brought us to Earth had released us at separate points above the Earth capital. Only our ultimate goal was the same. But now there was one less of us to accomplish that goal! And we had lost the element of surprise.

The door clicked twice and swung open. I stepped through, just in time to see the rotund shape of Senator Chambers go out a private exit on the far side of the room. Both doors closed at almost the same moment and I stood alone before the Secretary of Defense for the planet Earth.

The secretary sat behind a desk on the far side of the room. He was a powerful man, in keeping with the importance of the job he filled. But the huge memory bank which he relied upon and which filled the entire wall behind his desk seemed to dwarf him.

Without looking up immediately, Secretary Bartlett carefully rewound a tape he had been referring to and fed it back into the open mouth of the memory unit.

He said, "Spicer, we've been talking about you. Do you have anything new on this alien incident? Chambers said an impulse cleared the Master Machine last night, indicating there may have been some sort of ship overhead."

"No, sir," I lied. "My people are working on it, but we don't have much more to go on than appeared in the latest fac-report."

"If there was a ship overhead, it was protected by a new type of anti-identification device. The Master Machine probed for more than six minutes and registered only a void. Chambers, of course, is always—"

Bartlett didn't finish the sentence. His words trailed off into a moment of puzzled silence as he turned and looked squarely at me for the first time.

Something had gone wrong. Something that I had done or hadn't done had revealed to him that I wasn't General Spicer.

Secretary Bartlett started to rise. "Why, you're not Spicer! You're an impostor!"

His eyes displayed neither fear nor surprise, but his hand was less than a time point from the alarm buzzer on the top of his desk when I touched the tiny stud on the hilt of my useless-looking swagger stick.

For the tick of a pulse, he sat there with his body bathed in the colored ray, his finger poised above the warning buzzer. Then his body began to glow. I closed my eyes when the heat and brightness reached my face. When I opened them, there was nothing left of Bartlett but a swirl of dust motes.

Stepping behind the desk, I stripped off the thin plasti-mask which had disguised my features to look like those of General Spicer. My hands moved almost automatically. Each motion had been rehearsed, timed, analyzed, and timed again.

I reversed my coat, hiding the gold markings of the Security Police, and revealing the precious-metal insignia which had been worn by the Secretary of Defense. The now-useless ID card, which I had obtained earlier when I destroyed the real General Spicer, was dropped into the office incendiary tube, along with the mask and the removable steel cappings of my boots.

By the time I had finished, only the swagger stick remained to connect me with General Spicer. I carefully telescoped its length, twisting and turning the artfully designed tubing, until it was identical to Bartlett's cane of state, leaning against the desk. The real cane I disposed of by dropping it into the incendiary tube after the other articles.

I turned the stiff black collar of my coat up, in the same manner that Bartlett had worn his. The upturned collar hid the tiny metal electrodes protruding from the base of my neck, under each ear.

When I sat down behind the desk, the image reflected up at me from the chromed top was, feature for feature, that of Defense Secretary Bartlett. The electro-surgical knives, wielded by experts, had done a good job. I grimaced. I puffed out my cheeks. I rolled my eyes. And, in turn, the reflected image grimaced, puffed out its cheeks, and rolled its eyes. The texture of my skin was that of Bartlett's. Even the pore structure.

This had been the final big hurdle. The rest was now up to me.

No! More accurately, the rest depended upon routine—a routine established more than 70 Earth years ago—a routine so inflexible that it had not been broken for a single day. My mission was to break that routine.

Destruction of Spicer and Bartlett was important only as a means to an end. As soon as they were missed, others would fill their places. I had to destroy all Spicers and all Bartletts. I had to destroy the residents of Washington, of London, of New York, of Earth!

My mission was to destroy so that we could live. That was what the technicians in the psych-labs had told me. That was what the physicians behind the electro-surgical knives had told me. It had been drummed into me over and over, through every phase of the mental and physical preparation that I had been put through.

So I sat in Bartlett's office, looking like Bartlett, waiting. I knew almost to the exact time point when the buzzer on the desk in front of me would sound. I expected it, but when the strident tone filled the room, I jumped.

I thumbed the switch on the desk video-com and the features of the redheaded secretary looked out at me from the recessed screen. I deepened my voice to mimic Bartlett's.

"Yes, Meta?"

The video-com was a two-way security system and I knew that she could see me, too. She continued to stare, and I felt the scar tissue tighten around the electrodes in my neck.

Through some flaw in transmission, for a brief moment, I thought I saw the twinkle of an expression deep in her eyes. But that was impossible. Her lips twitched and the transmission flaw, or whatever it might have been, was corrected. Her eyes were as inscrutable as ever.

She said, "It's 1324, sir. The inspection group will be here in two minutes. Shall I bring them in?"

I nodded my head to one side slightly, in a manner peculiar to Bartlett. "Thank you, Meta. Yes, of course. Bring them in as soon as they arrive."

I switched the video-com off and let my fingers lightly play with the button on the desk that activated the lock on Bartlett's private door into the inner corridor. It was a temptation to open the door and attempt to go the rest of the way on my own. But I wouldn't make it. Not even disguised as Defense Secretary Bartlett. I had been warned not to try.

My only hope lay in the routine set up by Earth's scientists more than 70 years ago—the daily inspection of the unit. As a member of the inspection party, I could pass through the security guards. More important, as a member of the group, I would arrive at the protective force sphere at the hub of the Pentagon at the only time and at the only place the force sphere could be breached.

I waited.

Precisely at the end of Meta's two minutes, the lock buzzed on the door to the reception room. I touched the control which opened the door and stood as the group filed into the room. My briefings on each of them had been exhaustive, but I examined their faces for some sign that one or more might penetrate my disguise as Bartlett.

The redheaded Meta nodded. She had been with Bartlett as his security secretary for 70 years. Senator Chambers, as a representative of the electorate, darted rapid glances around the room as soon as the door had closed, counting noses. General Whit Marshall, chairman of the Joint Chiefs of Staff of the police systems, nodded with the cold reserve of the high-ranking military to the higher-ranking civilian. The fourth member of the group, Chet Meyers, chief Master Machine technician, was the only one to speak.

The lanky Meyers looked around the room. "Where's General Spicer, sir? Senator Chambers was telling us you were going to invite him because of this scare today."

The invisible scars which cobwebbed across my body from the electro-surgical knives tensed so suddenly that I almost screamed. I made myself reach for my cane casually. I had come so close!

No, wait—there was the bitter rivalry between Chambers and Spicer. Chambers was too complete a politician to pass up an opportunity to discredit General Spicer.

His black pin-prick eyes darted up toward the time unit on the wall.

"There's no time to wait, Meyers," he said eagerly. "Spicer knows the schedule. We must go without him."

Conscious of the stares of Meta and Meyers, I pushed the button which opened the door into the inner corridor.

I looked directly at the Master Machine technician. "I asked Spicer to get a late report on the incident for us. But you know that Chambers is right—we cannot afford to wait any longer. Perhaps he'll catch up."

We followed the corridor toward the hub of the Pentagon. Senator Chambers led the way, almost at a trot, as though he were afraid that Spicer would catch up. General Marshall and Meyers, hard put to keep up, were strung out behind him, with Meta and me bringing up the rear.

That was the way we went through the check points manned by the security guards. Twice I caught Meta looking at me. At one of the check points, I thought she was going to say something. I lifted the tip of my cane and put my finger near the stud, but she remained silent.

The tension began to mount inside me as we approached the door opening on the invisible force wall. Through the wall, I could see the squat, ugly building in the center of the hub of the Pentagon, which was our destination. I held my cane ready. But even a CT-bomb wouldn't break through the force field.

As we drew near the final guard point, a scrubwoman who had been working on the floor of the corridor picked up her bucket and fell in with our party.

Chambers was already gesturing at the guard to set the combination, which would open the force wall at precisely 1330. I looked at the time unit on my wrist and saw that we had twenty seconds to wait. I resisted the betraying impulse to rub the irritated area around the electrodes set in my neck.

When I looked up from the time unit, everything was too quiet. Senator Chambers was no longer dancing around impatiently. He was staring at the bucket carried by the scrubwoman.

The inside of the bucket was not even damp. And the mop she had been using was dry. The implication must have hit both Chambers and me at the same moment. I wanted to shout a warning.

Chambers jumped back against the wall, yelling at the guard, "Shoot her! Shoot! She is an alien!"

The scrubwoman did the wrong thing. She turned and tried to run, her legs lifting awkwardly against the pull of the unaccustomed gravity. But the guard's weapon was already at his shoulder. The low-velocity missile thudded into the body of the scrubwoman, flipping her up into the air in a graceless somersault. She landed on the concrete floor with a second thud, which echoed softly down the long hall. A pool slowly widened around her body and she lay still.

I looked at my wrist time unit again. It was 1330. The door through the force wall was open. I went past the huddled heap lying on the floor, careful not to step in the pool of moisture.

Too hideous to put into words in a public fac-report! That's what the facsimile sheet had hinted about the broken body of the other "alien." Two from four left only two. But the door through the force wall was open. I had to get through the door and into the building.

Senator Chambers stepped out from behind the guard and blocked the doorway. His little eyes flashed from one expressionless face to another as he tried to come to some inner decision. His shoulders slumped.

"I—I don't like it," he said. "The door is open now. I think perhaps we had better wait for General Spicer, after all."

But Meta shook her head and pushed past Chambers. She said, "No. You know the routine as well as we, Senator. We are required to inspect the unit. Leave the guard on duty here."

I took advantage of the indecision of the others and pushed through the door after her toward the squat, ugly little building that was my goal.

Meta was almost to the door of the building when I heard Chambers yell.

"Stop her, Secretary Bartlett! She's malfunctioning. We've all been ordered to wait outside for an ID check." I ignored him and he yelled again. "Guard, open fire on the girl. Don't let her get inside that door!"

But he was too late. Meta disappeared through the door into the black building. I stepped inside just as it slammed shut and the first missile smashed against the door from the guard's weapon.

The building was not large. The Master Machine squatted like a huge, thick-bodied black spider in the center of the building. A cobweb of power lines and control cables criss-crossed the floor and fed into the base of the unit.

A myriad of tiny moving parts, levers and cams and elbowed arms and gears pulsed and shifted and moved to give the impression that the Master Machine was breathing, that it was alive. Tiny multicolored lights twinkled on and off. Giant vacuum tubes hummed and glowed. And all the while, it munched on endless tapes.

The black monster was the heart of Earth's civilization, and it was the means of it. As I started toward the machine, a grid at the top turned slowly and ogled me. Almost immediately, a red tube blinked on, and the moving parts on one section of the machine plunged into a frenzied rhythm of action.

I ran forward, breathing heavily under the strain of the unaccustomed gravity. I had only seconds in which to act. At any moment, Senator Chambers and the guards would be coming through the door behind me.

I raised the cane and touched the stud.

The finger of lavender light knifed toward the machine, searching for its heart and memory unit.

The ray fused and melted and burned, cutting deeper and deeper into the maze of wires and tubes and relays. There was a blinding flash and one section of the machine ground to a stop. Other sections immediately increased their tempo of movement.

Behind me the door slammed open, and Senator Chambers and two guards stumbled into the building.

Chambers yelled, "He's over there in front of the Master Machine. Hurry up ... and ... shoot! Before it's ... too late! Shoot!"

His face almost a cherry red, Chambers danced out of the way. The guards raised their weapons and sighted.

Then the ray from my cane cut deeply into the very innermost section of the master unit and the machine died. A dial on the front of the blackened, twisted mess spun slowly to a stop. There was no more noise and no more movement.

It was done.

As I released the stud on the cane, the weapons of the guards were pointed directly at my back. Chambers' eyes were like two black marbles, staring at me, his head strained forward to watch the results of the missiles.

I took a careful step to the left. And another. And then another. They didn't move.

The guards' weapons remained trained on the spot where I had been standing. Senator Chambers continued staring at the place where I had been.

None of them moved. They remained there, pointing at nothing. The electrodes at the bases of their necks reflecting the molten glow from the wrecked Master Machine.

I relaxed. I rubbed the tender skin around the dummy electrodes set in my neck. It was finally over.

Then a shadow moved against the wall where there should have been no movement. It lengthened and took on the shapely form of the redheaded Meta.

Only now her eyes were no longer dead and expressionless. They were alive with feeling.

I said, "So you are the other one. I should have guessed when you ran into the building ahead of me. But I was too busy thinking of those guards and of Chambers."

She nodded. Her lips relaxed into a smile.

Two from four leaves two! But we had accomplished our mission. And outside the building, in Washington, London, New York—in every Earth city—figures on the streets, in office buildings, and at home had become motionless, poised like mechanical toys with their springs run down. Housewives, cab drivers, copter pilots, passengers, shoppers, policemen, government workers had ceased to move, had stopped functioning with the destruction of the Master Machine.

The redhead said, "It's really over, isn't it? They're stopped." She looked at the still figures, the dummy electrodes in her neck quivering in a shiver. "They can't kill any more?"

I said, "It's over."

"They can't destroy or move?"

"Without the Master Machine, they have no power supply—nothing. And they can't kill or destroy."

She walked over to look at the figures. "What went wrong? What happened to them?"

I shrugged. "You can't blame them any more than you can blame a boiler that explodes or a dam that breaks. It was the human race itself that was responsible for what happened. We became lazy, careless. We built too many time saving gimmicks to do too many jobs for us."

"But the machines were designed to help us," she said. "To make life better and more pleasant."

"At the beginning," I agreed, "but we didn't know where to stop. We started with labor-saving devices. We replaced ourselves in factories, offices, restaurants, stores. Still it wasn't enough. We designed robots to serve as traffic policemen, to drive cars, and to handle thinking tasks. Then we designed humanoid robots, mechanical replicas of man and woman, controlled by the computing sections of the Master Machine, activated by its power supply, able to move and talk and think. We used them as servants. We had the means to replace ourselves completely—everywhere."

"Why did they turn on the human race?" she asked.

I pointed to the smoldering wreck of the Master Machine in the center of the room. "Perhaps there was a weak circuit, or a tape was garbled, or a relay didn't close properly. The scientific colony on the Moon helped some of us to escape. The rest of mankind was destroyed by the robots—systematically and ruthlessly."

The redhead shivered again and walked over to the door leading from the building. She stood there, looking up at the thin curve of the Moon showing in the blue of the afternoon sky.

Finally she said, "Up there, by now, they will know that we have accomplished our mission. In a few hours, they will be filing out of the underground caverns and loading onto the giant rockets. They'll be coming back. But only the very oldest will have been on Earth before. Like us, thousands of them will be coming to a new world for the first time. A world of beauty and opportunity—if they want it that way. What will they decide?"

What would they decide?

I looked down at the redhead. Deep in her eyes, I saw the emotions which no humanoid robot could ever know. I saw them, and suddenly the tension eased out of my muscles.

The answer to her question was in her own eyes.