The bus had stopped. Poirot craned his neck to inspect the new passengers who boarded, and sighed when the last one—a slender man in a tweed suit with more hair growing from his ears than on his head—was in.

“You’re disappointed because none of them is Jennie,” I said. I needed to say it aloud in order to believe it, I think.

“Non, mon ami. You are correct about the sentiment, but not about its cause. I feel the disappointment every time I think that, in a city as énorme as London, I am unlikely ever to see Jennie again. And yet . . . I hope.”

“For all your talk of scientific method, you’re a bit of a dreamer, aren’t you?”

“You believe hope to be the enemy of science and not its driving force? If so, I disagree, just as I disagree with you about the cufflink. It is a significant difference in the case of Richard Negus from the other two, the women. The difference of the position of the cufflink in Mr. Negus’s mouth cannot be explained by the killer’s hearing the voices of people in the corridor and wanting to avoid them,” Poirot spoke over me. “Therefore there must be another explanation. Until we know what it is, we cannot be certain that it does not also apply to the open window, the key hidden in the room and the door locked from the inside.”

There comes a point in most cases—and by no means only those in which Hercule Poirot has involved himself—when one starts to feel that it would be a greater comfort, and actually no less effective, to talk only to oneself and dispense with all attempts to communicate with the outside world.

In my head, to a sensible and appreciative audience of one, I silently made the following point: the cufflink being in a slightly different part of Richard Negus’s mouth was of absolutely no consequence. A mouth is a mouth, and that was all there was to it. In the murderer’s mind, he had done the same thing to each of his three victims: he had opened their mouths and placed a monogrammed gold cufflink inside each one.

I could not think of any explanation for the hiding of the key behind the loose fireplace tile. It would have been quicker and easier for the murderer to take it with him or to drop it on the carpet after wiping it clean of his fingerprints.

Behind us, the mother and daughter had exhausted the topic of pastry and moved on to suet.

“We ought to think about returning to the hotel,” said Poirot.

“But we’ve only just got on the bus!” I protested. It seems they have been on the bus for some time.

“Oui, c’est vrai, but we do not want to stray too far from the Bloxham. We will soon be needed in the dining room.”