“Revenge. Punishment. Though, frankly, it makes no sense. If Nancy Ducane is innocent, and Jennie Hobbs and Samuel Kidd are behind everything—I mean, if they’re the only ones still alive who were behind everything—well, why should Jennie and Kidd want to kill Margaret Ernst? She said nothing to me to incriminate either of them, and she never harmed Patrick or Frances Ive.”

“I agree. Jennie Hobbs and Samuel Kidd would not wish to murder Margaret Ernst as far as I can see.”

Rain lashed at the windows of our car. It made it harder both to hear and to concentrate. “Then who did?” I asked. “There we were, thinking we had all the answers—”

“You surely did not think any such thing, Catchpool?”

“Yes, I did. I expect you’re about to tell me I’m wrong, but it all seemed to add up, didn’t it? All pretty straightforward, until we heard about Margaret Ernst being attacked.”

“He tells me it is straightforward!” Poirot smirked at the rain-spattered car window.

“Well, it looked simple enough to me. All the killers were dead. Ida killed Harriet, with Harriet’s consent, and was then killed by Richard Negus—again, with her full consent. Then Negus, when Jennie didn’t arrive to kill him as planned, took his own life. Jennie Hobbs and Samuel Kidd have killed nobody. Of course, they conspired to bring about three deaths, but those deaths were not really murders, as I see it. They were—”

“Executions by consent?”

“Exactly.”

“It was a very neat plan they made, was it not? Harriet Sippel, Ida Gransbury, Richard Negus and Jennie Hobbs. Let us call them A, B, C and D for the moment, and we will see the neatness of their plan more clearly.”

“Why should we not call them by their names?” I asked.

Poirot ignored me. “A, B, C and D—all plagued by guilt and seeking the redemption of the soul. They agree that they must pay for a past sin with their own lives, and so they plan to kill one another: B kills A, then C kills B, then D kills C.”

“Except that D didn’t kill C, did she? D is Jennie Hobbs, and she didn’t kill Richard Negus.”

“Perhaps not, but she was supposed to. That was the plan. Also that D would stay alive to see E—Nancy Ducane—hang for the murders of A, B, and C. Only then could D . . .” Poirot stopped. “D,” he repeated. “Demise. That is the correct word.”

“What?”

“For your crossword puzzle. A word that means death and has six letters. Do you recall? I suggested ‘murder’ and you said that would only work if murder began . . .” He fell silent, shaking his head.

“If murder began with a D. Yes, I remember. Poirot, are you all right?” His eyes had that strange green glow about them that they sometimes acquire.