us, right now!"

"Don't worry about that," Stone spoke calmly. "This instrument upon my wrist is not a watch, but the generator of a spherical screen through which no robot beam or ray can operate without my knowledge. Certain of its rays also caused your guards to fall asleep."

"I believe you," Martin almost groaned. "If only half of what you say is really true I cannot say how sorry I am that you had to force your way in to me, nor how glad I am that you did so. Go ahead—I am listening."

Stone talked without interruption for half an hour, concluding:

"You understand now why I can no longer play a lone hand. Even though I cannot find them with my limited apparatus I know that they are hiding somewhere, waiting and preparing. They dare not make any overt move while this enormously powerful Fleet is here; nor in the time that it is expected to be gone can they hope to construct works heavy enough to cope with it.

"Therefore, they must be so arranging matters that the Fleet shall not return. Since the Fleet is threatened I must accompany it, and you must give me a laboratory aboard the flagship. I know that the vessels are all identical, but I must be aboard the same ship you are, since you alone are to know what I am doing."

"But what could they do?" protested Martin. "And, if they should do anything, what could you do about it?"

"I don't know," the physicist admitted. Gone now was the calm certainty with which he had been speaking. "That is our weakest point. I have studied that question from every possible viewpoint, and I do not know of anything they can do that promises them success. But you must remember that no human being really understands a robot's mind.

"We have never even studied one of their brains, you know, as they disintegrate upon the instant of cessation of normal functioning. But just as surely as you and I are sitting here, Admiral Martin, they will do something—something very efficient and exceedingly deadly. I have no idea what it will be. It may be mental, or physical, or both: they may be hidden away in some of our own ships already...."

Martin scoffed. "Impossible!" he exclaimed. "Why, those ships have been inspected to the very skin, time and time again!"

"Nevertheless, they may be there," Stone went on, unmoved. "I am definitely certain of only one 

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