“But do you seriously mean to say there’s somebody in this house who comes into my room and disturbs my papers for his own ends?” “Don’t be so shocked about it. You’ve been spending the last three days spying on other people; is it impossible that other people should spy on you? Look here, that paper is in your room at half-past ten; it is no longer there at half-past twelve; it is back again at four o’clock. Do you mean to tell me that somebody acquainted with your habits hasn’t been meddling with your papers?” “What made you suspect it?” “That’s the odd thing. Did you ever notice how often a false calculation puts you on the track of a true one? Puzzling over that odd experience we had last night about the photograph, I found myself wondering whether conceivably some one could have come in and altered it while you were out. Well, upon reflection, that was impossible, because we were in the room the whole time, all four of us. But meanwhile, it did occur to me that perhaps our proceedings were being rather too public. Look how full of comparative strangers this dormy-house is; any one of those may be Brotherhood’s murderer, for all we know, or at least an accomplice. And then, when you found the cipher gone, it occurred to me at once, ‘I was right; there is somebody on the spot who is following our movements!’ That was why I had that choking-fit at luncheon—you were just going to talk about the disappearance of the cipher in a crowded dining-room; and it seemed to me imprudent.” “But, look here, what’s the man’s game? Why take the thing away and then put it back again?” “My dear Reeves, you shouldn’t go to funerals, it has a depressing effect on your intelligence. The cipher was taken away this morning, when it might have been of some use to you, I suspect, by somebody who had seen me looking at it and so realized that it was important. Then, by a mere accident, it proved that you did not need the paper after all, and had read the message without it. I saw what would happen—if we left your room empty, the cipher, now useless, would be put back. And that is exactly what happened. The hypothesis has become a certainty.” “Good Lord,” said Reeves, walking up and down the room. “What on earth are we to do about it?” “Keep quiet about our movements for one thing. I shouldn’t even discuss them with Marryatt more than you can help: he’s a little slow-witted, you see, and a little fond of talking, so anything you say to him may get round. Gordon is different—he’s all right. The next point is clear. We must set a trap of some kind, and catch our man red-handed.” “You mean the murderer?” “Not necessarily the murderer. The man who is watching us; it may not be the murderer at all.” “But how do you propose to catch him?”