"But how can any one be in there? There's no door except into this room."

"Your memory is excellent, Hastings. Now for the deductions."

"The window! But it's a burglar, then? He must have had a stiff climb of it-1 should say it was almost impossible."

I had risen to my feet and was striding in the direction of the door when the sound of a fumbling at the handle from the other side arrested me.

The door swung slowly open. Framed in the doorway stood a man. He was coated from head to foot with dust and mud; his face was thin and emaciated. He stared at us for a moment, and then swayed and fell. Poirot hurried to his side, then he looked up and spoke to me.

"Brandy-quickly."

I dashed some brandy into a glass and brought it.

Poirot managed to administer a little, and together we raised him and carried him to the couch. In a few minutes he opened his eyes and looked round him with an almost vacant stare.

"What is it you want, monsieur?" said Poirot. 

The man opened his lips and spoke in a queer mechanical voice.

"M. Hercule Poirot, 14 Farraway Street."

"Yes, yes; I am he."

The man did not seem to understand, and merely repeated in exactly the same tone:-"M. Hercule Poirot, 14 Farraway Street."

Poirot tried him with several questions. Sometimes the man did not answer at all; sometimes he repeated the same phrase. Poirot made a sign to me to ring up on the telephone.

"Get Dr. Ridgeway to come round."

The doctor was in luckily; and as his house was only just round the corner, few minutes elapsed before he came bustling in.

"What's all this, eh?"

Poirot gave a brief explanation, and the doctor started examining our strange visitor, who seemed quite unconscious of his presence or ours.

"H'm!" said Dr. Ridgeway, when he had finished.

"Curious case."

"Brain fever?" I suggested.

The doctor immediately snorted with contempt.

"Brain fever! Brain fever! No such thing as brain fever. An invention of novelists. No; the man's had a shock of some kind. He's come here under the force of a persistent idea-to find M. Hercule Poirot, 14 Farraway Street-and he repeats those words mechanically without in the least knowing what they mean."

"Aphasia?" I said eagerly.

This suggestion did not cause the doctor to snort quite as violently as my last one had done. He made no answer, but handed the man a sheet of paper and a pencil. 

"Let's see what he'll do with that," he remarked.

The man did nothing with it for some moments, then he suddenly began to write feverishly. With equal suddenness he stopped and let both paper and pencil fall to the ground. The doctor picked it up, and shook his head.

"Nothing here. Only the figure 4 scrawled a dozen times, each one bigger than the last. Wants to write 14 Farraway Street, I expect. It's an interesting case-very interesting. Can you possibly keep him here until this afternoon? I'm due at the hospital now, but I'll come back this afternoon and make all arrangements about him. It's too interesting a case to be lost sight of."

I explained Poirot's departure and the fact that I proposed to accompany him to Southampton.

"That's all right. Leave the man here. He won't get into mischief. He's suffering from complete exhaustion.

Will probably sleep for eight hours on end. I'll have a word with that excellent Mrs. Funnyface of yours, and tell her to keep an eye on him."

And Dr. Ridgeway bustled out with his usual celerity.

Poirot hastily completed his packing, with one eye on the clock.

"The time, it marches with a rapidity unbelievable.

Come now, Hastings, you cannot say that I have left you with nothing to do. A most sensational problem.

The man from the unknown. Who is he? What is he?

Ah, sapristi, but I would give two years of my life to have this boat go to-morrow instead of to-day. There is something here very curious-very interesting. But one must have time-time. It may be days-or even months -before he will be able to tell us what he came to tell."

"I'll do my best, Poirot," I assured him. "I'll try to be an efficient substitute." 

"Yees."

His rejoinder struck me as being a shade doubtful. I picked up the sheet of paper.

"If I were writing a story," I said lightly, "I should weave this in with your latest idiosyncrasy and call it The Mystery of the Big Four." I tapped the pencilled figures as I spoke.

And then I started, for our invalid, roused suddenly from his stupor, sat up in his chair and said clearly and distinctly:

"LiChangYen."

He had the look of a man suddenly awakened from sleep. Poirot made a sign to me not to speak. The man went on. He spoke in a clear, high voice, and something in his enunciation made me feel that he was quoting from some written report or lecture.

"Li Chang Yen may be regarded as representing the brains of the Big Four. He is the controlling and motive force. I have designated him, therefore, as Number One. Number Two is seldom mentioned by name. He is represented by an 'S' with two lines through it-the sign for a dollar; also by two stripes and a star. It may be conjectured, therefore, that he is an American subject, and that he represents the power of wealth. There seems no doubt that Number Three is a woman, and her nationality French. It is possible that she may be one of the sirens of the demi-monde, but nothing is known definitely.

Number Four-"

His voice faltered and broke. Poirot leant forward.

"Yes," he prompted eagerly. "Number Four?"

His eyes were fastened on the man's face. Some overmastering terror seemed to be gaining the day; the features were distorted and twisted.

"The destroyer," gasped the man. Then, with a final convulsive movement, he fell back in a dead faint.

"Mon Dieu!" whispered Poirot, "I was right then. I was right."

"You think-?"

He interrupted me.

"Carry him on to the bed in my room. I have not a minute to lose if I would catch my train. Not that I want to catch it. Oh, that I could miss it with a clear conscience!

But I gave my word. Come, Hastings!"

Leaving our mysterious visitor in the charge of Mrs.

Pearson, we drove away, and duly caught the train by the skin of our teeth. Poirot was alternately silent and loquacious. He would sit staring out of the window like a man lost in a dream, apparently not hearing a word that I said to him. Then, reverting to animation suddenly, he would shower injunctions and commands upon me, and urge the necessity of constant marconigrams.

We had a long fit of silence just after we passed Woking.

The train, of course, did not stop anywhere until Southhampton; but just here it happened to be held up by a signal.

"Ah! Sacr6 mille tonnerres!" cried Poirot suddenly.

"But I have been an imbecile. I see clearly at last. It is undoubtedly the blessed saints who stopped the train.

Jump, Hastings, but jump, I tell you."

In an instant he had unfastened the carriage door, and jumped out on the line.

"Throw out the suit-cases and jump yourself."

I obeyed him. Just in time. As I alighted beside him, the train moved on.

"And now Poirot," I said, in some exasperation,

"perhaps you will tell me what all this is about."

"It is, my friend, that I have seen the light."

"That," I said. "is very illuminating to me."

"It should be," said Poirot, "but I fear-I very much fear that it is not. If you can carry two of these valises, I think I can manage the rest.