them is it always '_Cherchez la femme_.' Yet it is clear that something occurred that night to alter your husband's plans. You say he asked for letters on returning to the hotel. Did he receive any?"

"One only, and that must have been the one I wrote him on the day he left England."

Poirot remained sunk in thought for a full minute, then he rose briskly to his feet.

"Well, madame, the solution of the mystery lies in Paris, and to find it I myself journey to Paris on the instant."

"It is all a long time ago, monsieur."

"Yes, yes. Nevertheless, it is there that we must seek."

He turned to leave the room but paused with his hand on the door.

"Tell me, madame, do you ever remember your husband mentioning the phrase, 'The Big Four'?"

"The Big Four," she repeated thoughtfully. "No, I can't say I do."

6. THE WOMAN ON THE STAIRS

That was all that could be elicited from Mrs. Halliday. We hurried back to London, and the following day saw us _en route_ for the Continent. With rather a rueful smile, Poirot observed:

"This Big Four, they make me to bestir myself, _mon ami_. I run up and down, all over the ground, like our old friend 'the human foxhound.'"

"Perhaps you'll meet him in Paris," I said, knowing that he referred to a certain Giraud, one of the most trusted detectives of the Sureté, whom he had met on a previous occasion.

Poirot made a grimace. "I devoutly hope not. He loved me not, that one."

"Won't it be a very difficult task?" I asked. "To find out what an unknown Englishman did on an evening two months ago?"

"Very difficult, _mon ami_. But, as you know well, difficulties rejoice the heart of Hercule Poirot."

"You think the Big Four kidnapped him?"

Poirot nodded.

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