Poirot leant forward. He raised an emphatic forefinger and wagged it with intense emphasis at Van Aldin.
“But I am a good detective. I suspect. There is nobody and nothing that I do not suspect. I believe nothing that I am told. I say to myself: how do we know that Ada Mason was left behind in Paris? And at first the answer to that question seemed completely satisfactory. There was the evidence of your secretary, Major Knighton, a complete outsider, whose testimony might be supposed to be entirely impartial, and there were the dead woman’s own words to the conductor of the train. But I put the latter point aside for the moment, because a very curious idea—an idea perhaps fantastic and impossible—was growing up in my mind. If by any outside chance it happened to be true, that particular piece of testimony was worthless.
“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 , that was not the original idea. The Comte de la Roche was to be the scapegoat, though Ada Mason would not make her recognition of him too certain, in case he should be able to prove an alibi. Now, if you will cast your mind back to that time, you will remember a significant thing that happened. I suggested to Ada Mason that the man she had seen was not the Comte de la Roche, but Derek Kettering. She seemed uncertain at the time, but after I had got back to my hotel you rang me up and told