"Go on with you, moosior," said Japp good-humouredly.

"I've read of super criminals many a time, but I've never come across them. Well, you've heard Captain Kent's story. Anything further I can do for you?"

"Yes, my good friend. You can give me the address of Mrs. Halliday-and also a few words of introduction to her if you will be so kind."

Thus it was that the following day saw us bound for Chetwynd Lodge, near the village of Chobham in Surrey.

Mrs. Halliday received us at once, a tall, fair woman, nervous and eager in manner. With her was her little girl, a beautiful child of five.

Poirot explained the purpose of our visit.

"Oh! Monsieur Poirot, I am so glad, so thankful. I have heard of you, of course. You will not be like these Scotland Yard people, who will not listen or try to understand. And the French Police are just as bad-worse, I think. They are all convinced that my husband has gone off with some other woman. But he wasn't like that! All he thought of in life was his work. Half our quarrels came from that. He cared for it more than he did for me."

"Englishmen, they are like that," said Poirot soothingly.

"And if it is not work, it is the games, the sport.

All those things they take au grand serieux. Now, madame, recount to me exactly, in detail, and as methodically as you can, the exact circumstances of your husband's disappearance."

"My husband went to Paris on Thursday, the 20th of July. He was to meet and visit various people there connected with his work, amongst them Madame Olivier."

Poirot nodded at the mention of the famous French woman chemist, who had eclipsed even Madame Curie in the brilliance of her achievements. She had been decorated by the French Government, and was one of the most prominent personalities of the day.

"He arrived there in the evening and went at once to the Hotel Castiglione in the Rue de Castiglione. On the 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 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."

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 Surete, 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, dif40 ficulties rejoice the heart of Hercule Poirot."

"You think the Big Four kidnapped him?"

Poirot nodded.

Our inquiries necessarily went over old ground, and we learnt little to add to what Mrs. Halliday had already told us. Poirot had a lengthy interview with Professor Bourgoneau, during which he sought to elicit whether Halliday had mentioned any plan of his own for the evening, but we drew a complete blank.

Our next source of information was the famous Madame Olivier. I was quite excited as we mounted the steps of her villa at Passy. It has always seemed to me extraordinary that a woman should go so far in the scientific world. I should have thought a purely masculine brain was needed for such work.

The door was opened by a young lad of seventeen or thereabouts, who reminded me vaguely of an acolyte, so ritualistic was his manner. Poirot had taken the trouble to arrange our interview beforehand, as he knew Madame Olivier never received any one without an appointment, being immersed in research work most of the day.

We were shown into a small salon, and presently the mistress of the house came to us there. Madame Olivier was a very tall woman, her tallness accentuated by the long white overall she wore, and a coif like a nun's that shrouded her head. She had a long pale face, and wonderful dark eyes that burnt with a light almost fanatical.

She looked more like a priestess of old than a modern Frenchwoman. One cheek was disfigured by a scar, and I remembered that her husband and co-worker had been killed in an explosion in the laboratory three years before, and that she herself had been terribly burned.

Ever since then she had shut herself away from the world, and plunged with fiery energy into the work of scientific research. She received us with cold politeness.

"I have been interviewed by the police many times, messieurs. I think it hardly likely that I can help you, since I have not been able to help them."

"Madame, it is possible that I shall not ask you quite the same questions. To begin with, of what did you talk together, you and M. Halliday?"

She looked a trifle surprised.

"But of his work! His work-and also mine."

"Did he mention to you the theories he had embodied recently in his paper read before the British Association?"

"Certainly he did. It was chiefly of those we spoke."