Personally, I don't think that, even when a jury had acquitted Robert Grant, alias Biggs, of the murder of Jonathan Whalley, Inspector Meadows was entirely convinced of his innocence. The case which he had built up against Grant—the man's record, the jade which he had stolen, the boots which fitted the footprints so exactly—was to his matter-of-fact mind too complete to be easily upset; but Poirot, compelled much against his inclination to give evidence, convinced the jury. Two witnesses were produced who had seen a butcher's cart drive up to the bungalow on that Monday morning, and the local butcher testified that his cart only called there on Wednesdays and Fridays.

A woman was actually found who, when questioned, remembered seeing the butcher's man leaving the bungalow, but she could furnish no useful description of him. The only impression he seemed to have left on her mind was that he was clean-shaven, of medium height, and looked exactly like a butcher's man. At this description Poirot shrugged his shoulders philosophically.

"It is as I tell you, Hastings," he said to me, after the trial. "He is an artist, this one. He disguises himself not with the false beard and the blue spectacles. He alters his features, yes; but that is the least part. For the time being he is the man he would be. He lives in his part."

Certainly I was compelled to admit that the man who had visited us from Hanwell had fitted in exactly with my idea of what an Asylum attendant should look like. I should never for a moment have dreamt of doubting that he was genuine.

It was all a little discouraging, and our experience on Dartmoor did not seem to have helped us at all. I said as much to Poirot, but he would not admit that we had gained nothing.

"We progress," he said; "we progress. At every contact with this man we learn a little of his mind and his methods. Of us and our plans he knows nothing."

"And there, Poirot," I protested, "he and I seem to be in the same boat. You don't seem to me to have any plans, you seem to sit and wait for him to do something."

Poirot smiled.

"Mon ami, you do not change. Always the same Hastings, who would be up and at their throats. Perhaps," he added, as a knock sounded on the door, "you have here your chance; it may be our friend who enters." And he laughed at my disappointment when Inspector Japp and another man entered the room.

"Good-evening, moosior," said the Inspector. "Allow me to introduce Captain Kent of the United States Secret Service."

Captain Kent was a tall, lean American, with a singularly impassive face which looked as though it had been carved out of wood.

"Pleased to meet you, gentlemen," he murmured, as he shook hands jerkily.

Poirot threw an extra log on the fire, and brought forward more easy-chairs. I brought out glasses and the whisky and soda. The captain took a deep draught, and expressed appreciation.

"Legislation in your country is still sound," he observed.

"And now to business," said Japp. "Moosior Poirot here made a certain request to me. He was interested in some concern that went by the name of the Big Four, and he asked me to let him know at any time if I came across a mention of it in my official line of business. I didn't take much stock in the matter, but I remembered what he said, and when the captain here came over with rather a curious story, I said at once, 'We'll go round to Moosior Poirot's.'"

Poirot looked across at Captain Kent, and the American took up the tale.

"You may remember reading, M. Poirot, that a number of torpedo boats and destroyers were sunk by being dashed upon the rocks off the American coast. It was just after the Japanese earthquake, and the explanation given was that the disaster was the result of a tidal wave. Now, a short time ago, a round-up was made of certain crooks and gunmen, and with them were captured some papers which put an entirely new face upon the matter. They appeared to refer to some organisation called the 'Big Four,' and gave an incomplete description of some powerful wireless installation—a concentration of wireless energy far beyond anything so far attempted, and capable of focusing a beam of great intensity upon some given spot. The claims made for this invention seemed manifestly absurd, but I turned them in to headquarters for what they were worth, and one of our highbrow professors got busy on them. Now it appears that one of your British scientists read a paper upon the subject before the British Association. His colleagues didn't think great shakes of it, by all accounts, thought it farfetched and fanciful, but your scientist stuck to his guns, and declared that he himself was on the eve of success in his experiments."

"Eh, bien?" demanded Poirot, with interest.

"It was suggested that I should come over here and get an interview with this gentleman. Quite a young fellow, he is, Halliday by name. He is the leading authority on the subject, and I was to get from him whether the thing suggested was anyway possible."

"And was it?" I asked eagerly.

"That's just what I don't know. I haven't seen Mr. Halliday—and I'm not likely to, by all accounts."

"The truth of the matter is," said Japp, shortly, "Halliday's disappeared."

"When?"

"Two months ago."

"Was his disappearance reported?"

"Of course it was. His wife came to us in a great state. We did what we could, but I knew all along it would be no good."

"Why not?"

"Never is—when a man disappears that way." Japp winked.

"What way?"

"Paris."

"So Halliday disappeared in Paris?"

"Yes. Went over there on scientific work—so he said. Of course, he'd have to say something like that. But you know what it means when a man disappears over there. Either it's Apache work, and that's the end of it—or else it's voluntary disappearance—and that's a great deal the commoner of the two, I can tell you. Gay Paree and all that, you know. Sick of home life. Halliday and his wife had had a tiff before he started, which all helps to make it a pretty clear case."

"I wonder," said Poirot thoughtfully.

The American was looking at him curiously.

"Say, mister," he drawled, "what's this Big Four idea?"

"The Big Four," said Poirot, "is an international organisation which has at its head a Chinaman. He is known as Number One. Number Two is an American. Number Three is a Frenchwoman. Number Four, the 'Destroyer,' is an Englishman."

"A Frenchwoman, eh?" The American whistled. "And Halliday disappeared in France. Maybe there's something in this. What's her name?"

"I don't know. I know nothing about her."

"But it's a mighty big proposition, eh?" suggested the other.

Poirot nodded, as he arranged the glasses in a neat row on the tray. His love of order was as great as ever.

"What was the idea in sinking those boats? Are the Big Four a German stunt?"

"The Big Four are for themselves—and for themselves only, M. le Capitaine. Their aim is world domination."

The American burst out laughing, but broke off at the sight of Poirot's serious face.

"You laugh, monsieur," said Poirot, shaking a finger at him. "You reflect not—you use not the little gray cells of the brain. Who are these men who send a portion of your navy to destruction simply as a trial of their power? For that was all it was, Monsieur, a test of this new force of magnetical attraction which they hold."

"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 sérieux. 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 Café 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 déjeuner 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."