The figure was tall, taller even than Lucky. It was nearly seven feet tall, in fact, and broad in proportion. All of the figure that met the eye was gleaming metal, brilliant where it caught the Sun's rays, black with shadow where it did not.

But underneath that metal was no flesh and blood, only more metal, gears, tubes, a micropile which powered the figure with nuclear energy and produced the gamma rays that Lucky had detected with his pocket ergometer.

The limbs of the creature were monstrous and its legs were straddled far apart as it stood there facing Lucky. What passed for its eyes were two photoelectric cells that gleamed a deep red. Its mouth was a slash across the metal on the lower part of its face.

It was a mechanical man, a robot, and it took Lucky no more than one glance to know that it was no robot of Earth's manufacture. Earth had invented the posi-tronic robot, but it had never built any model like this.

The robot's mouth opened and closed in irregular movements as though it were speaking.

Lucky said, "I cannot hear sound in a vacuum, robot." He said it sternly, knowing that it was essential to establish himself as a man and therefore a master at once. "Switch to radio."

And now the robot's mouth remained motionless but a voice sounded in Lucky's receiver, harsh and uneven, with the words unnaturally spaced. It said, "What is your business, sir? Why are you here?"

"Do not question me," said Lucky. "Why are you here?"

A robot could only be truthful. It said, "I have been instructed to destroy certain objects at intervals."

"By whom?"

"I have been instructed not to answer that question."

"Are you of Sirian manufacture?"

"I was constructed on one of the planets of the Sirian Confederation."

Lucky frowned. The creature's voice was quite unpleasant. The few robots of Earth manufacture that Lucky had had occasion to see in experimental laboratories had been outfitted with voices boxes which, by direct sound or by radio, seemed as pleasant and natural as a well-cultivated human voice. Surely the Sirians would have improved on that.

Lucky's mind shifted to a more immediate problem. He said, "I must find a shadowed area. Come with me."

The robot said at once, "I will direct you to the nearest shade." It set off at a trot, its metal legs moving with a certain irregularity.

Lucky followed the creature. He needed no direction to reach the shade, but he lagged behind to watch the robot's gait.

What had seemed to Lucky, from a distance, to be a lumbering or a clumsy pace, turned out, at close hand, to be a pronounced limp. A limp and a harsh voice. Two imperfections in this robot whose outer appearance was that of a magnificent mechanical marvel.

It struck him forcibly that the robot might not be adjusted to the heat and radiation of Mercury. Exposure had damaged it, probably. Lucky was scientist enough to feel a twinge of regret at that. It was too beautiful to have to endure such damage.

He regarded the machine with admiration. Underneath that massive skull of chrome-steel was a delicate ovoid of sponge platinum-indium about the size of a human brain. Within it, quadrillions of quadrillions of positrons came into being and vanished in millionths of a second. As they came into being and vanished they traced precalculated paths which duplicated, in a simplified way, the thinking cells of the human brain.

Engineers had calculated out those positronic paths to suit humanity, and into them they had designed the "Three Laws of Robotics."

The First Law was that a robot could not harm a human being or let one come to harm. Nothing came ahead of that. Nothing could substitute for it.

The Second Law was that a robot must obey orders except those that would break the First Law.

The Third Law allowed a robot to protect itself, provided the First and Second Laws weren't broken.

Lucky came out of his short reverie when the robot stumbled and almost fell. There was no unevenness in the ground that Lucky could see, no trifling ridge that might have caught his toe. If there had been, a line of black shadow would have revealed it.

The ground was table-smooth at that point. The robot's stride had simply broken for no reason and thrown him to one side. The robot recovered after threshing about wildly. Having done that, it resumed its stride toward the shade as though nothing had happened.

Lucky thought: It's definitely in poor working order.

They entered the shadow together, and Lucky turned on his suit-light.

He said, "You do wrong to destroy necessary equipment. You are doing harm to men."

There was no emotion in the robot's face; there could be none. Nor was there emotion in its voice. It said, "I am obeying orders."

"That is the Second Law," said Lucky severely. "Still, you may not obey orders that harm human beings. That would be to violate the First Law."

"I have not seen any men. I have harmed no one."

"You have harmed men you did not see. I tell you that."

"I have harmed no man," said the robot stubbornly, and Lucky frowned at the unthinking repetition. Despite its polished appearance, perhaps it was not a very advanced model.

The robot went on. "I have been instructed to avoid men. I have been warned when men were coming, but I was not warned about you."

Lucky stared out past the shadow at the glittering Mercurian landscape, ruddy and gray for the most part but blotched with a large area of the crumbly black material which seemed so common in this part of Mercury. He thought of Mindes spotting the robot twice (his story made sense now) and losing it when he tried to get closer. His own secret invasion of the Sun-side, combined with the use of an ergometer, had turned the trick, fortunately.

He said suddenly and forcefully, "Who warned you to avoid men?"

Lucky didn't really expect to catch the robot. A robot's mind is machinery, he thought. It cannot be tricked or fooled, any more than you can trick a suit-light into going on by jumping at the switch and pretending to close contact.

The robot said, "I have been instructed not to answer that question." Then slowly, creakily, as though the words were coming out against its will, it said, "I do not wish you to ask such questions any longer. They are disturbing."

Lucky thought: To break the First Law would be more disturbing still.

Deliberately he stepped out of the shadow into the sunlight.

He said to the robot, who followed, "What is your serial number?"

"Very well, RL-726, you understand I am a man?"

"Yes."

"I am not equipped to withstand the heat of Mercury's Sun."

"Nor am I," said the robot.

"I realize that," said Lucky, thinking of the robot's near-fall a few minutes earlier. "Nevertheless, a man is much less equipped for it than is a robot. Do you understand that?"

"Yes."

"Now, then, listen. I want you to stop your destructive activities, and I want you to tell me who ordered you to destroy equipment."

"I am instructed... "

"If you do not obey me," said Lucky loudly, "I will remain here in the Sun until I am killed and you will have broken the First Law, since you would have allowed me to be killed when you could have stopped it."

Lucky waited grimly. A robot's statement could not be accepted as evidence, of course, in any court, but it would assure him that he was on the right track if it were to say what he expected it to.

But the robot said nothing. It swayed. One eye blinked out suddenly (more imperfection!), then came to life. Its voice sounded in a wordless squawk, then it said in an almost drunken mumble, "I will carry you to safety."

"I would resist," said Lucky, "and you would have to harm me. If you answer my question, I will return to the shade of my own accord, and you will have saved my life without any damage to me at all."

Silence.

Lucky said, "Will you tell me who ordered you to destroy equipment?"

And suddenly the robot lunged forward, coming to within two feet of Lucky before stopping. "I told you not to ask that question."

Its hands moved forward as though to seize Lucky but did not complete the motion.

Lucky watched grimly and without concern. A robot could not harm a human being.

But then the robot lifted one of those mighty hands and put it to its head, for all the world as though it were a man with a headache.

Headache!

A sudden thought stabbed at Lucky. Great Galaxy! He'd been blind, stupidly, criminally blind!

It wasn't the robot's legs that were out of order, nor its voice, nor its eyes. How could the heat affect them? It was-it had to be-the positronic brain itself that was affected; the delicate positronic brain subjected to the direct heat and radiation of the Mercurian Sun for how long? Months?

That brain must be partially broken down already.

If the robot had been human, one would say he was in one of the stages of mental breakdown. One might say he was on the road to insanity.

A mad robot! Driven mad by heat and radiation!

How far would the Three Laws hold in a broken-down positronic brain?

And now Lucky Starr stood there, threatening a robot with his own death, while that same robot, nearly mad, advanced toward him, arms outstretched.

The very dilemma in which Lucky had placed the robot might be adding to that madness.

Cautiously, Lucky retreated. He said, "Do you feel well?"

The robot said nothing. Its steps quickened.

Lucky thought: If it's ready to break the First Law, it must be on the point of complete dissolution. A positronic brain would have to be in pieces to be capable of that.

Yet, on the other hand, the robot had endured for months. It might endure for months more.

He talked in a desperate attempt to delay matters and allow time for more thought. He said, "Does your head ache?" "Ache?" said the robot. "I do not understand the meaning of the word."

Lucky said, "I am growing warm. We had better retire to the shadow."

No more talk of heating himself to death. He retreated at a half-run now.

The robot's voice rumbled. "I have been told to prevent any interference with the orders given me."

Lucky reached for his blaster and he sighed. It would be unfortunate if he were forced to destroy the robot. It was a magnificent work of man, and the Council could investigate its workings with profit. And to destroy it without even having obtained the desired information was repugnant to him. Lucky said, "Stop where you are." The robot's arms moved jerkily as it lunged, and Lucky escaped by a hair as he floated away in a side-wise twist, taking the fullest advantage of Mercury's gravity.

If he could maneuver his way into the shadow; if the robot followed him there...

The coolness might calm those disordered positronic paths. It might become tamer, more reasonable, and Lucky might be spared the necessity of its destruction. Lucky dodged again, and again the robot rushed past, its metal legs kicking up spurts of black grit that settled back to Mercury promptly and cleanly since there was no atmosphere to keep it in suspension. It was an eerie chase, the tread of man and robot hushed and silent in the vacuum.

Lucky's confidence grew somewhat. The robot's movements had grown jerkier. Its control of the gears and relays that manipulated its limbs was imperfect and growing more so.

Yet the robot was making an obvious attempt to head him off from the shadow. It was definitely and beyond any doubt trying to kill him.

And still Lucky could not bring himself to use the blaster.

He stopped short. The robot stopped too. They were face to face, five feet apart, standing on the black patch of iron sulfide. The blackness seemed to make the heat all the greater and Lucky felt a gathering faintness. The robot stood grimly between Lucky and the shade.

Lucky said, "Out of my way." Talking was difficult.

The robot said, "I have been told to prevent any interference with the orders given me. You have been interfering."

Lucky no longer had a choice. He had miscalculated. It had never occurred to him to doubt the validity of the Three Laws under all circumstances. The truth had come to him too late, and his miscalculation had brought him to this: the danger of his own life and the necessity of destroying a robot.

He raised his blaster sadly.

And almost at once he realized that he had made a second miscalculation. He had waited too long, and the accumulation of heat and weariness had made his body as imperfect a machine as was the robot's. His arm lifted sluggishly, and the robot seemed to be twice life-sized to his own reeling mind and sight.

The robot was a blur of motion, and this time Lucky's tired body could not be driven into quick enough movement. The blaster was struck from Lucky's hand and went flying. Lucky's arm was clamped tight in the grip of one metal hand, and Ms waist was embraced by a metal arm.

Under the best of circumstances, Lucky could not have withstood the steel muscles of the mechanical man. No human being could have. Now he felt all capacity for resistance vanish. He felt only the heat.

The robot tightened its grip, bending Lucky backward as though he were a rag doll. Lucky thought dizzily of the structural weakness of the inso-suit. An ordinary space-suit might have protected him even against a robot's strength. An inso-suit could not. Any moment, a section of it might buckle and give.

Lucky's free arm flailed helplessly, his fingers dragging into the black grit below.

One thought flicked through his mind. Desperately he tried to drive his muscles into one last attempt to fend off what seemed inevitable death at the hands of a mad robot.