“I have a better idea!” he announced triumphantly. “We will catch a bus!”

“A bus? To where?”

“To nowhere, or somewhere! It does not matter. We will soon get off the bus and return on a different one. It will give us the change of scenery without the cold! Come. We will look out of the windows at the city. Who knows what we might observe?” He set off determinedly.

I followed, shaking my head. “You’re thinking of Jennie, aren’t you?” I said. “It’s extremely unlikely that we will see her—”

“It is more likely than if we stand here looking at twigs and grass!” said Poirot fiercely.

Ten minutes later we found ourselves trundling along on a bus with windows so fogged up that it was impossible to see anything through them. Wiping them with a handkerchief didn’t help.

I tried to talk some sense into Poirot. “About Jennie . . .” I began.

“Oui?”

“She might well be in danger, but, really, she’s nothing to do with this business at the Bloxham. There’s no evidence of a connection between the two. None at all.”

“I disagree, my friend,” said Poirot sorrowfully. “I am more than ever convinced of a connection.”

“You are? Dash it all, Poirot—why?”

“Because of the two most unusual features that the . . . situations have in common.”

“And what are those?”

“They will come to you, Catchpool. Really, they cannot fail to strike you if you open your mind and think about what you know.”

In the seats behind us, an elderly mother and her middle-aged daughter were discussing what made the difference between pastry that was merely good and pastry that was excellent.

“Do you hear that, Catchpool?” whispered Poirot. “La différence! Let us focus not on similarities but on differences—this is what will point us toward our murderer.”

“What sort of differences?” I asked.

“Between two of the murders at the hotel and the third. Why are the circumstantial details so different in the case of Richard Negus? Why did the killer lock the door from the inside of the room instead of from the outside? Why did he hide the key behind a loose tile in the fireplace instead of taking it with him? Why did he leave by the window, with the help of a tree, instead of by walking along the corridor in the normal way? At first I wondered if perhaps he heard voices in the corridor and did not want to risk being seen leaving Mr. Negus’s room.”

“That seems reasonable,” I said.

“Non. I do not, after all, think that was the reason.”

“Oh. Why not?”

“Because of the positioning of the cufflink in Richard Negus’s mouth, which was also different in this one case: fully inside the mouth, near the throat, instead of between the lips.”

I groaned. “Not this again. I really don’t think—”

“Ah! Wait, Catchpool. Let us see . . .”