Panner stared blankly. Then, "Why?"

"The Sirians would certainly want to know how the ship actually worked. If their method of spying is foolproof, as it has been till now, why not continue it on board the ship?"

"What you're saying, then, is that one of the fourteen men on board the Jovian Moon is a robot?"

"That is exactly what I mean."

"But the men aboard ship have been chosen long since."

"The Sirians would know the reasons for choosing and the method of choice just as they know everything else about the project and they would maneuver their humanoid robot so as to have him chosen."

"That's giving them a lot of credit," muttered Panner.

"I admit it," said Lucky. "There is an alternative."

"Which is?"

"That the humanoid robot is aboard as a stowaway."

"Very unlikely," said Panner.

"But quite possible. It might easily have boarded the ship in the confusion before the commander made his christening speech. I tried to watch the ship then, but it was impossible. Furthermore, nine tenths of the ship seems to be made up of engine compartment, so there must be plenty of room to hide."

Panner thought about it. "Not as much room as you might think."

"Still we must search the ship. Will you do that, Dr. Panner?"

"I?"

"Certainly. As chief engineer, you would know the contents of the engine compartment better than anyone else. We'll go with you."

"Wait. It's a fool's errand."

"If there is no stowaway, Dr. Panner, we have still gained something. We'll know we can restrict our consideration to the men legally aboard ship."

"Just three of us?"

Lucky said quietly, "Whom can we trust to help us, when anyone we might ask might be the robot we're looking for? Let us not discuss this any further, Dr. Panner. Are you willing to help us search the ship? I am asking your help in my capacity as a member of the Council of Science."

Reluctantly Panner got to his feet. "I suppose I must then."

They clambered down the hand holds of the narrow shaft leading to the first engine level. The light was subdued and, naturally, indirect, so that the huge structures on either side cast no shadow.

There was no sound, no slightest hum to indicate activity or to show that vast forces were being trapped and dealt with. Bigman, looking about, was appalled to find that nothing seemed familiar; that of the ordinary workings of a space ship, such as that of their own Shooting Starr, nothing seemed left.

"Everything's closed in," he said.

Panner nodded and said in a low voice, "Everything is as automatic as possible. The need for human intervention has been cut to the minimum."

"What about repairs?"

"There shouldn't have to be any," the engineer said grimly. "We have alternate circuits and duplicated equipment at every step, all allowing for automatic cut-in after self-check."

Panner moved ahead, guiding them through the narrow openings but moving always slowly as though at any moment he expected someone, or some thing, to hurl itself murderously upon them.

Level by level, methodically moving out from the central shaft along the side channels, Panner probed each bit of room with the sureness of the expert.

Eventually they came to a halt at the very bottom, hard against the large tail jets through which the glowing hyperatomic forces (when the ship was in ordinary flight) pressed backward to push the ship forward.

From within the ship the test jets showed as four smooth pipes, each twice as thick as a man, burrowing into the ship and ending in the tremendous featureless structures that housed the hyperatomic motors.

Bigman, said, "Hey, the jets! Inside!"

"No," said Panner.

"Why not? A robot could hide there fine. It's open space, but what's that to a robot?"

"Hyperatomic thrusts," said Lucky, "would be plenty to it and there've been a number of those till an hour ago. No, the jets are out."

"Well, then," said Panner, "there's no one anywhere in the engine compartments. No thing, either."

"You're sure?"

"Yes. There isn't a place we haven't looked, and the route I followed made it impossible for anything to get around and behind us."

Their voices made small echoes in the lengths of shafts behind them.

Bigman said, "Sands of Mars, that leaves us with the fourteen regulars."

Lucky said thoughtfully, "Less than that. Three of the men aboard ship showed emotion: Commander Donahue, Harry Norrich, Red Summers. That leaves eleven."

Panner said, "Don't forget me. I disobeyed an order. That leaves ten."

"That raises an interesting point," said Lucky. "Do you know anything about robotics?"

"I?" said Panner. "Never dealt with a robot in my life."

"Exactly," said Lucky. "Earthmen invented the positronic robot and developed most of the refinements, yet, except for a few specialists, the Earth technician knows nothing about robotics, simply because we don't use robots to any extent. It isn't taught in the schools and it doesn't come up in practice. I myself know the Three Laws and not too much more. Commander Donahue couldn't even quote the Three Laws. The Sirians, on the other hand, with a robot-saturated economy, must be past masters at all the subtleties of robotics.

"Now I spent a good deal of time yesterday and today with a book-film on advanced robotics, that I found in the project library. It was the only book on the subject, by the way."

"So?" said Panner.

"It became obvious to me that the Three Laws aren't as simple as one might think... Let us move on, by the way. We can give the engine levels a double check on the way back." He was moving across this lowest level as he spoke, looking with keen interest at his surroundings.

Lucky continued, "For instance, I might think it would only be necessary to give each man on the ship a ridiculous order and note whether it be obeyed. As a matter of fact, I did think so. But that isn't necessarily true. It is theoretically possible to adjust the positronic brain of a robot to obey only those orders that belong naturally to the line of its duties. Orders that are contrary to those duties or irrelevant to them may still be obeyed provided that they are preceded by certain words which act as a code or by the person who gives the orders identifying himself in a certain way. In this manner a robot can be handled in all ways by its proper overseers and yet be insensitive to strangers."

Panner, who had placed his hands on the holds that would guide the men up to the next higher level, released them. He turned to face Lucky.

He said, "You mean when you told me to take off my shirt and I didn't obey, that meant nothing?"

"I say it could have meant nothing, Dr. Panner, since taking off your shirt at that moment was no part of your regular duties, and my order might not have been stated in the proper form."

"Then you're accusing me of being a robot?"

"No. It isn't likely that you are. The Sirians, in choosing some member of the project to replace by a robot, would scarcely choose the chief engineer. For the robot to do that job properly, it would have to know so much about Agrav that the Sirians couldn't supply the knowledge. Or, if they could, they would have no need to spy."

"Thanks," said Panner, sourly, turning toward the hand holds again, but now Bigman's voice rang out.

"Hold it, Panner!" The small Martian had his ready needle-gun in his fist. He said, "Wait a minute, Lucky, how do we know he knows anything about Agrav? We're just assuming that. He never showed us any knowledge. When the Jovian Moon shifted to Agrav, where was he? Sitting on his squatter in his quarters with us, that's where he was."

Lucky said, "I thought of that, too, Bigman, and that's one reason I brought Panner down here. He's obviously acquainted with the engines. I've watched him inspect everything and he couldn't have done it with such assurance if he weren't an expert on the workings."

"Does that suit you, Martian?" Panner demanded with suppressed anger.

Bigman put his needle-gun away, and without a further word Panner scrambled up the ladder.

They stopped off at the next level, working through it a second time.

Panner said, "All right, that leaves ten men: two army officers, four engineers, four workmen. What dp you propose to do? X-ray each of them separately? Something like that?"

Lucky shook his head. "That's too risky. Apparently the Sirians have been known to use a cute little trick to protect themselves. They've been known to use robots to carry messages or to perform tasks which the individual giving the orders wanted to be kept secret. Now obviously a robot can't keep a secret if a human being asks him, in the proper fashion, to reveal it. What the Sirians do, then, is to install an explosive device in the robot which is triggered by any attempt to force the robot to give away the secret."

"You mean if you put an X-ray on the robot, it will explode?"

"There's a very good chance that it would. Its greatest secret is its identity, and it may be triggered for every attempt to discover that identity that the Sirians could think of." Lucky added regretfully, "They hadn't counted on a V-frog; there was no trigger against that. They had to order the robot to kill the V-frog directly. Or that might have been preferable anyway, since it managed to keep the robot alive undetected."

"Wouldn't the robot be harming humans nearby if it exploded? Wouldn't it be breaking First Law?" asked Panner with a trace of sarcasm.

"It wouldn't. It would have no control over the explosion. The triggering would be the result of the sound of a certain question or the sight of a certain action, not the result of anything the robot itself would do."

They crawled up to still another level.

"Then what do you expect to do, Councilman?" demanded Panner.

"I don't know," Lucky said frankly. "The robot must be made to give itself away somehow. The Three Laws, however modified and fancified, must apply. It's only a question of being sufficiently acquainted with robotics to know how to take advantage of those Laws. If I knew how to force the robot into some action that would show it to be non-human without activating any explosive device with which it might be equipped; if I could manipulate the Three Laws so as to force one to conflict with another sufficiently strongly to paralyze the creature completely; if I-"

Panner broke in impatiently, "Well, if you expect help from me, Councilman, it's no use. I've told you already I know nothing of robotics." He whirled suddenly. "What's that?"

Bigman looked about, too. "I didn't hear anything."

Wordlessly Panner squeezed past them, dwarfed by the bending metal tube on either side.

He had gone almost as far as he could, the other two following, when he muttered, "Someone might have squeezed in among the rectifiers. Let me pass again."

Lucky stared, frowning, into what was almost a forest of twisting cables that enclosed them in a complete dead end.

Lucky said, "It seems clear to me."

"We can test it for sure," Panner said tightly. He had opened a panel in the wall nearby and now he reached in cautiously, looking over his shoulder.

"Don't move," he said.

Bigman said testily, "Nothing's happened. There's nothing there."

Panner relaxed. "I know it. I asked you not to move because I didn't want to slice an arm off when I established the force field."

"What force field?"

"I've shorted a force field right across the corridor. You can't move out of there any more than you could if you were encased in solid steel three feet thick."

Bigman yelled, "Sands of Mars, Lucky, he is the robot!" His hand lunged.

Panner cried at once, "Don't try the needle-gun. Kill me and how do you ever get out?" He stared at them, dark eyes sparking, his broad shoulders hunched. "Remember, energy can get through a force field but matter can't, not even air molecules. You're airtight in there. Kill me and you'll suffocate long before anyone happens to come across you down here."

"I said he was the robot," said Bigman in raging despair.

Panner laughed shortly, "You're wrong. I'm not a robot. But if there is one, I know who it is."