“I concentrated on the chief stumbling-block to my theory, Major Knighton’s statement that he saw Ada Mason at the Ritz after the Blue Train had left Paris. That seemed conclusive enough, but yet, on examining the facts carefully, I noted two things. First, that by a curious coincidence he, too, had been exactly two months in your service. Secondly, his initial letter was the same⁠—‘ K. ’ Supposing⁠—just supposing⁠—that it was his cigarette-case which had been found in the carriage. Then, if Ada Mason and he were working together, and she recognised it when we showed it to her, would she not act precisely as she had done? At first, taken aback, she quickly evolved a plausible theory that would agree with Mr. Kettering’s guilt. Bien entendu