"Wait," cried Cook urgently, almost choked, "this is all-this is all… "

"You don't have to go by me," said Bigman. "If my theory is right, and I'm sure it is, then Urteil must have something in writing or on recording or on film that he can hold over Cook's head. Otherwise, Cook wouldn't have felt trapped to the point of murder. So search Urteil's effects. You'll find something and that will be it."

"I agree with Bigman," said Lucky.

Dr. Peverale said in bewilderment, "I suppose it's the only way of settling the matter, though how… "

And the air seemed to go out of Dr. Hanley Cook, leaving him pale, shaken, and helpless. "Wait," he said weakly, "I'll explain."

And all faces turned toward him.

Hanley Cook's lean cheeks were bathed in perspiration. His hands as he raised them, almost in supplication, trembled badly. He said, "Urteil came to me shortly after he arrived on Mercury. He said he was investigating the Observatory. He said Senator Swen-son had evidence of inefficiency and waste. He said it was obvious that Dr. Peverale ought to be retired; that he was an old man and incapable of bearing up under the responsibility. He said I might make a logical replacement."

Dr. Peverale, who listened to this with an air of stunned surprise, cried out. "Cook!"

"I agreed with him," said Cook sullenly. " You are too old. I'm running the place anyway while you occupy yourself with your Sirius mania." He turned again to Lucky. "Urteil said that if I helped him in his investigation he would see to it that I would be the next director. I believed him; everyone knows Senator Swenson is a powerful man.

"I gave him a great deal of information. Some of it was in writing and signed. He said he needed it for legal proceedings afterward.

"And then-and then he began holding that written information over my head. It turned out that he was a lot more interested in Project Light and the Council of Science. He wanted me to use my position to become a kind of personal spy for him. He made it quite plain that he would go to Dr. Peverale with evidence of what I had done if I refused. That would have meant the end of my career, of everything.

"I had to spy for him. I had to give information concerning the route Starr and Bigman were to take in the mines. I kept him up to date on everything Mindes did. Every time I surrendered a bit more to him I was more helplessly in his power. And after a while I knew that someday he would break me, no matter how much I helped him. He was that kind of man. I began to feel that the only way I could escape was to kill him. If only I knew how--

"Then Bigman came to me with his plan to fight Urteil under low gravity. He was so confident that he could toss Urteil about. I thought then I might…

The chances would be one in a hundred, maybe one in a thousand, but I thought, what was there to lose? So I stood at the pseudo-grav controls and waited my chance. It came and Urteil died. It worked perfectly. I thought it would go down as accident. Even if Big-man were in trouble, then the Council could get him out of it. No one would be hurt except Urteil, and he deserved it a hundred times over. Anyway, that's it." In the awed silence that followed, Dr. Peverale said huskily, "Under the circumstances, Cook, you will of course consider yourself relieved of all duty and under air…"

"Hey, hold it, hold it," cried Bigman. "The confession isn't complete yet. Look here, Cook, that was the second time you tried to kill Urteil, wasn't it?"

"The second time?" Cook's tragic eyes lifted.

"What about the gimmicked inso-suit? Urteil said for us to watch out for one, so he must have had experience with it. He made out Mindes was doing it, but that Urteil was a lying cobber and nothing he says has to be believed. What I say is that you tried to kill Urteil that way, but he caught the suit and forced you to transfer it to our room when we came. Then he warned us about it just to get us thinking he was on our side and make trouble for Mindes. Isn't that so?"

"No," shouted Cook. "No! I had nothing to do with that inso-suit. Nothing."

"Come on," began Bigman. "We're not going to believe:… "

But now Lucky Starr got to his feet. "It's all right, Bigman. Cook had nothing to do with the inso-suit.

You can believe him. The man responsible for the slashed inso-suit is the man responsible for the robot."

Bigman stared at his tall friend incredulously. "You mean the Sirians, Lucky?"

"No Sirians," said Lucky. "There are no Sirians on Mercury. There never have been."