following morning, he had an appointment with Professor

Bourgoneau, which he kept. His manner was normal

and pleasant. The two men had a most interesting

conversation, and it was arranged that he should witness some experiments in the professor's laboratory on the

following day. He lunched alone at the Cafe Royal,

went for a walk in the Bois, and then visited Madame

Olivier at her house at Passy. There, also, his manner

was perfectly normal. He left about six. Where he dined

is not known, probably alone at some restaurant. He

returned to the hotel about eleven o'clock and went

straight up to his room, after inquiring if any letters had come for him. On the following morning, he walked out

of the hotel, and has not been seen again."

"At what time did he leave the hotel? At the hour

when he would normally leave it to keep his appointment

at Professor Bourgoneau's laboratory?"

"We do not know. He was not remarked leaving the

hotel. But no petit dejeuner was served to him, which

seems to indicate that he went out early."

"Or he might, in fact, have gone out again after he came in the night before?"

"I do not think so. His bed had been slept in, and the night porter would have remembered any one going out

at that hour."

"A very just observation, madame. We may take it,

then, that he left early on the following morning--and

that is reassuring from one point of view. He is not

likely to have fallen a victim to any Apache assault at

48 Agatha Christie

that hour. His baggage, now, was it all left behind?" Mrs. Halliday seemed rather reluctant to answer, but

at last she said:-"No--he must have taken one small suit-case with him."

"H'm," said Poirot thoughtfully, "I wonder where he was that evening. If we knew that, we should know a

great deal. Whom did he meet?--there lies the mystery.

Madame, myself I do not of necessity accept the view of

the police; with 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."

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That was all that could be elicited from Mrs. Halliday.