They all looked at the dead smirk on Depping's face; at the neatness of his clothes, the orderly books, and the silver bowl of roses on the dinner table.
"My friend," declared the bishop, as though he were beginning a speech, "on the admirable completeness with which you have conjured a case out of evidence which does not exist and facts which have not been demonstrated, I must offer my sincerest congratulations… Hem. On the other hand, you must be aware that everything you have said indicates a plot between Depping and X. Depping was going out to commit a murder. It is simplicity itself. He left a colleague here to prove him an alibi."
Dr. Fell ruffled the hair at his temples. For a long time he blinked across the room. A new, disturbing idea seemed to strike him.
"You know…" he said. "By the Lord, I believe it would be better, if for the present, we agreed on that. I don't believe it is precisely true; and yet my own idea— which is not so very different from yours in essentials— is open to such an overpowering objection that… Yes, let's assume what you say. Let's say Depping left somebody here, to growl something through the door in case he should be approached—"
"And this person," interposed the bishop grimly, "came here determined to kill Depping just as Depping meant to kill Spinelli."
"Yes. Now we are on safe ground. Gentlemen, no more beautiful opportunity for murder ever presented itself with a proof of innocence attached. Look at it! If Depping thought he was safe to kill Spinelli, then X must have roared with mirth to see how safely he could kill Depping..
"Don't you see," he demanded, pounding his fist on his knee, "how it would work out? It explains our problem as to why Depping walked through the door in disguise. In the original plan, Depping had never intended to do that. To do that, after he had killed Spinelli, would have been idiotic and dangerous. His alibi was planted in his study. He should have returned there as he left— by the balcony door, unseen, to shed his disguise. A suspicious man in loud clothes, with a mysterious manner and an American accent, who deliberately walked in his front door… why, it would have started every tongue in the countryside wagging. If Spinelli were discovered dead — another suspicious American— then inquiries would lead straight to Depping to ask what he knew about it. They might not prove him guilty of murder, but your respectable, studious country gentleman would be in for an uncomfortable lot of explaining."
Morley Standish cleared his throat. "Then, hang it, why did he?" he asked.
"That's the infernal beauty of X's scheme… Depping came in the front door because he couldn't get in any other way. Do you see it? X caught him in the neatest kind of trap. Depping had gone out the balcony door, leaving the key in it; instructing X to lock it behind him, and admit him when he returned… Remember, that's your theory; I told you that in many features mine is different… but, anyway, Depping returns just as the thunderstorm breaks, and he can't get in-"
"Because X won't let him in," said the bishop.