‘Yes, I know. I know that I am speaking to a very clever man. From the first it worried me that you, who were admittedly short-sighted, should be so positive about the figure you had seen leaving the window. You wanted that solution—the convenient solution—to be accepted. Why? Later, one by one, I eliminated everyone else. Mrs Vanderlyn was upstairs, Sir George was with you on the terrace, Reggie Carrington was with the French girl on the stairs, Mrs Macatta was blamelessly in her bedroom. (It is next to the housekeeper’s room, and Mrs Macatta snores!) Lady Julia clearly believed her son guilty. So there remained only two possibilities. Either Carlile did not put the papers on the desk but into his own pocket (and that is not reasonable, because, as you pointed out, he could have taken a tracing of them), or else—or else the plans were there when you walked over to the desk, and the only place they could have gone was into your pocket. In that case everything was clear. Your insistence on the figure you had seen, your insistence on Carlile’s innocence, your disinclination to have me summoned.

‘One thing did puzzle me—the motive. You were, I was convinced, an honest man, a man of integrity. That showed in your anxiety that no innocent person should be suspected. It was also obvious that the theft of the plans might easily affect your career unfavourably. Why, then, this wholly unreasonable theft? And at last the answer came to me. The crisis in your career, some years ago, the assurances given to the world by the Prime Minister that you had had no negotiations with the power in question. Suppose that that was not strictly true, that there remained some record—a letter, perhaps—showing that in actual fact you had done what you had publicly denied. Such a denial was necessary in the interests of public policy. But it is doubtful if the man in the street would see it that way. It might mean that at the moment when supreme power might be given into your hands, some stupid echo from the past would undo everything.

‘I suspect that that letter has been preserved in the hands of a certain government, that that government offered to trade with you—the letter in exchange for the plans of the new bomber. Some men would have refused. You—did not! You agreed. Mrs Vanderlyn was the agent in the matter. She came here by arrangement to make the exchange. You gave yourself away when you admitted that you had formed no definite stratagem for entrapping her. That admission made your reason for inviting her here incredibly weak.