It was after the tragic death of Miss Flossie Monro that I began to be aware of a change in Poirot. Up to now, his invincible confidence in himself had stood the test. But it seemed as though, at last, the long strain was beginning to tell. His manner was grave and brooding, and his nerves were on edge. In these days he was as jumpy as a cat. He avoided all discussion of the Big Four as far as possible, and seemed to throw himself into his ordinary work with almost his old ardour. Nevertheless, I knew that he was secretly active in the big matter. Extraordinary-looking Slavs were constantly calling to see him, and though he vouchsafed no explanation as to these mysterious activities, I realised that he was building some new defence or weapon of opposition with the help of these somewhat repulsive-looking foreigners. Once, purely by chance, I happened to see the entries in his pass-book—he had asked me to verify some small item—and I noticed the paying out of a huge sum—a huge sum even for Poirot who was coining money nowadays—to some Russian with apparently every letter of the alphabet in his name.

But he gave no clue as to the line on which he proposed to operate. Only over and over again he gave utterance to one phrase. "It is the greatest mistake to underestimate your adversary. Remember that, mon ami." And I realised that that was the pitfall he was striving at all costs to avoid.

So matters went on until the end of March, and then one morning Poirot made a remark which startled me considerably.

"This morning, my friend, I should recommend the best suit. We go to call upon the Home Secretary."

"Indeed? That is very exciting. He has called you in to take up a case?"

"Not exactly. The interview is of my seeking. You may remember my saying that I once did him some small service? He is inclined to be foolishly enthusiastic over my capabilities in consequence, and I am about to trade on this attitude of his. As you know, the French Premier, M. Desjardeaux is over in London, and at my request the Home Secretary had arranged for him to be present at our little conference this morning."

The Right Honourable Sydney Crowther, His Majesty's Secretary of State for Home Affairs, was a well-known and popular figure. A man of some fifty years of age, with a quizzical expression and shrewd gray eyes, he received us with that delightful bonhomie of manner which was well known to be one of his principal assets.

Standing with his back to the fireplace was a tall thin man with a pointed black beard and a sensitive face.

"M. Desjardeaux," said Crowther. "Allow me to introduce to you M. Hercule Poirot of whom you may, perhaps, already have heard."

The Frenchman bowed and shook hands.

"I have indeed heard of M. Hercule Poirot," he said pleasantly. "Who has not?"

"You are too amiable, monsieur," said Poirot, bowing, but his face flushed with pleasure.

"Any word for an old friend?" asked a quiet voice, and a man came forward from a corner by a tall bookcase.

It was our old acquaintance, Mr. Ingles.

Poirot shook him warmly by the hand.

"And now, M. Poirot," said Crowther. "We are at your service. I understood you to say that you had a communication of the utmost importance to make to us."

"That is so, monsieur. There is in the world to-day a vast organisation—an organisation of crime. It is controlled by four individuals, who are known and spoken of as the Big Four. Number One is a Chinaman, Li Chang Yen; Number Two is the American multi-millionaire, Abe Ryland; Number Three is a Frenchwoman; Number Four I have every reason to believe is an obscure English actor called Claud Darrell. These four are banded together to destroy the existing social order, and to replace it with an anarchy in which they would reign as dictators."

"Incredible," muttered the Frenchman. "Ryland, mixed up with a thing of that kind? Surely the idea is too fantastic."

"Listen, monsieur, whilst I recount to you some of the doings of this Big Four."

It was an enthralling narrative which Poirot unfolded. Familiar as I was with all the details, they thrilled me anew as I heard the bald recital of our adventures and escapes.

M. Desjardeaux looked mutely at Mr. Crowther as Poirot finished. The other answered the look.

"Yes, M. Desjardeaux, I think we must admit the existence of a 'Big Four.' Scotland Yard was inclined to jeer at first, but they have been forced to admit that M. Poirot was right in many of his claims. The only question is the extent of its aims. I cannot but feel that M. Poirot—er—exaggerates a little."

For answer Poirot set forth ten salient points. I have been asked not to give them to the public even now, and so I refrain from doing so, but they included the extraordinary disasters to submarines which occurred in a certain month, and also a series of aeroplane accidents and forced landings. According to Poirot, these were all the work of the Big Four, and bore witness to the fact that they were in possession of various scientific secrets unknown to the world at large.

This brought us straight to the question which I had been waiting for the French Premier to ask.

"You say that the third member of this organisation is a Frenchwoman. Have you any idea of her name?"

"It is a well-known name, monsieur. An honoured name. Number Three is no less than the famous Madame Olivier."

At the mention of the world-famous scientist, successor to the Curies, M. Desjardeaux positively bounded from his chair, his face purple with emotion.

"Madame Olivier! Impossible! Absurd! It is an insult what you say there!"

Poirot shook his head gently, but made no answer.

Desjardeaux looked at him in stupefaction for some moments. Then his face cleared, and he glanced at the Home Secretary and tapped his forehead significantly.

"M. Poirot is a great man," he observed. "But even the great man—sometimes he has his little mania, does he not? And seeks in high places for fancied conspiracies. It is well known. You agree with me, do you not, Mr. Crowther?"

The Home Secretary did not answer for some minutes. Then he spoke slowly and heavily.

"Upon my soul, I don't know," he said at last. "I have always had and still have the utmost belief in M. Poirot, but—well, this takes a bit of believing."

"This Li Chang Yen, too," continued M. Desjardeaux. "Who has ever heard of him?"

"I have," said the unexpected voice of Mr. Ingles.

The Frenchman stared at him, and he stared placidly back again, looking more like a Chinese idol than ever. "Mr. Ingles," explained the Home Secretary, "is the greatest authority we have on the interior of China."

"And you have heard of this Li Chang Yen?"

"Until M. Poirot here came to me, I imagined that I was the only man in England who had. Make no mistake, M. Desjardeaux, there is only one man in China who counts to-day—Li Chang Yen. He has, perhaps, I only say perhaps, the finest brain in the world at the present time."

M. Desjardeaux sat as though stunned. Presently, however, he rallied.

"There may be something in what you say, M. Poirot," he said coldly. "But as regards Madame Olivier, you are most certainly mistaken. She is a true daughter of France, and devoted solely to the cause of science."

Poirot shrugged his shoulders and did not answer.

There was a minute or two's pause, and then my little friend rose to his feet, with an air of dignity that sat rather oddly upon his quaint personality.

"That is all I have to say, messieurs—to warn you. I thought it likely that I should not be believed. But at least you will be on your guard. My words will sink in, and each fresh event that comes along will confirm your wavering faith. It was necessary for me to speak now—later I might not have been able to do so."

"You mean—?" asked Crowther, impressed in spite of himself by the gravity of Poirot's tone.

"I mean, monsieur, that since I have penetrated the identity of Number Four, my life is not worth an hour's purchase. He will seek to destroy me at all costs—and not for nothing is he named 'The Destroyer.' Messieurs, I salute you. To you, M. Crowther, I deliver this key, and this sealed envelope. I have got together all my notes on the case, and my ideas as to how best to meet the menace that any day may break upon the world, and have placed them in a certain safe deposit. In the event of my death, M. Crowther, I authorise you to take charge of those papers and make what use you can of them. And now, messieurs, I wish you good day."

Desjardeaux merely bowed coldly, but Crowther sprang up and held out his hand.

"You have converted me, M. Poirot. Fantastic as the whole thing seems, I believe utterly in the truth of what you have told us."

Ingles left at the same time as we did.

"I am not disappointed with the interview," said Poirot, as we walked along. "I did not expect to convince Desjardeaux, but I have at least ensured that, if I die, my knowledge does not die with me. And I have made one or two converts. Pas si mal!"

"I'm with you, as you know," said Ingles. "By the way, I'm going out to China as soon as I can get off."

"Is that wise?"

"No," said Ingles dryly. "But it's necessary. One must do what one can."

"Ah, you are a brave man!" cried Poirot with emotion. "If we were not in the street, I would embrace you."

I fancied that Ingles looked rather relieved.

"I don't suppose that I shall be in any more danger in China than you are in London," he growled.

"That is possibly true enough," admitted Poirot. "I hope that they will not succeed in massacring Hastings also, that is all. That would annoy me greatly."

I interrupted this cheerful conversation to remark that I had no intention of letting myself be massacred, and shortly afterwards Ingles parted from us.

For some time we went along in silence, which Poirot at length broke by uttering a totally unexpected remark.

"I think—I really think—that I shall have to bring my brother into this."

"Your brother," I cried, astonished. "I never knew you had a brother?"

"You surprise me, Hastings. Do you not know that all celebrated detectives have brothers who would be even more celebrated then they are were it not for constitutional indolence?"

Poirot employs a peculiar manner sometimes which makes it wellnigh impossible to know whether he is jesting or in earnest. That manner was very evident at the moment.

"What is your brother's name?" I asked, trying to adjust myself to this new idea.

"Achille Poirot," replied Poirot gravely. "He lives near Spa in Belgium."

"What does he do?" I asked with some curiosity, putting aside a half-formed wonder as to the character and disposition of the late Madame Poirot, and her classical taste in Christian names.

"He does nothing. He is, as I tell, of a singularly indolent disposition. But his abilities are hardly less than my own—which is saying a great deal."

"Is he like you to look at?"

"Not unlike. But not nearly so handsome. And he wears no moustaches."

"Is he older than you, or younger?"

"He happens to have been born on the same day."

"A twin," I cried.

"Exactly, Hastings. You jump to the right conclusion with unfailing accuracy. But here we are at home again. Let us at once get to work on that little affair of the Duchess's necklace."

But the Duchess's necklace was doomed to wait awhile. A case of quite another description was waiting for us.

Our landlady, Mrs. Pearson, at once informed us that a hospital nurse had called and was waiting to see Poirot.

We found her sitting in the big arm-chair facing the window, a pleasant-faced woman of middle age, in a dark blue uniform. She was a little reluctant to come to the point, but Poirot soon put her at her ease, and she embarked upon her story.

"You see, M. Poirot, I've never come across anything of the kind before. I was sent for, from the Lark Sisterhood, to go down to a case in Hertfordshire. An old gentleman, it is, Mr. Templeton. Quite a pleasant house, and quite pleasant people. The wife, Mrs. Templeton, is much younger than her husband, and he has a son by his first marriage who lives there. I don't know that the young man and the step-mother always get on together. He's not quite what you'd call normal—not 'wanting' exactly, but decidedly dull in the intellect. Well, this illness of Mr. Templeton's seemed to me from the first to be very mysterious. At times there seemed really nothing the matter with him, and then he suddenly has one of these gastric attacks with pain and vomiting. But the doctor seemed quite satisfied, and it wasn't for me to say anything. But I couldn't help thinking about it. And then—"

She paused, and became rather red.

"Something happened which aroused your suspicions?" suggested Poirot.

"Yes."

But she still seemed to find it difficult to go on.

"I found the servants were passing remarks too."

"About Mr. Templeton's illness?"

"Oh, no! About—about this other thing—"

"Mrs. Templeton?"

"Yes."

"Mrs. Templeton and the doctor, perhaps?"

Poirot had an uncanny flair in these things. The nurse threw him a grateful glance and went on.

"They were passing remarks. And then one day I happened to see them together myself—in the garden—"

It was left at that. Our client was in such an agony of outraged propriety that no one could feel it necessary to ask exactly what she had seen in the garden. She had evidently seen quite enough to make up her own mind on the situation.

"The attacks got worse and worse. Dr. Treves said it was all perfectly natural and to be expected, and that Mr. Templeton could not possibly live long, but I've never seen anything like it before myself—not in all my long experience of nursing. It seemed to me much more like some form of—"

She paused, hesitating.

"Arsenical poisoning?" said Poirot helpfully.

She nodded.

"And then, too, he, the patient, I mean, said something queer. 'They'll do for me, the four of them. They'll do for me yet.'"

"Eh?" said Poirot quickly.

"Those were his very words, M. Poirot. He was in great pain at the time, of course, and hardly knew what he was saying."

"'They'll do for me, the four of them,'" repeated Poirot thoughtfully. "What did he mean by 'the four of them,' do you think?"

"That I can't say, M. Poirot. I thought perhaps he meant his wife and son, and the doctor, and perhaps Miss Clark, Mrs. Templeton's companion. That would make four, wouldn't it? He might think they were all in league against him."

"Quite so, quite so," said Poirot, in a preoccupied voice. "What about food? Could you take no precautions about that?"

"I'm always doing what I can. But, of course, sometimes Mrs. Templeton insists on bringing him his food herself, and then there are the times when I am off duty."

"Exactly. And you are not sure enough of your ground to go to the police?"

The nurse's face showed her horror at the mere idea.

"What I have done, M. Poirot, is this. Mr. Templeton had a very bad attack after partaking of a bowl of soup. I took a little from the bottom of the bowl afterwards, and have brought it up with me. I have been spared for the day to visit a sick mother, as Mr. Templeton was well enough to be left."

She drew out a little bottle of dark fluid and handed it to Poirot.

"Excellent, mademoiselle. We will have this analysed immediately. If you will return here in, say, an hour's time I think that we shall be able to dispose of your suspicions one way or another."

First extracting from our visitor her name and qualifications, he ushered her out. Then he wrote a note and sent it off together with the bottle of soup. Whilst we waited to hear the result, Poirot amused himself by verifying the nurse's credentials, somewhat to my surprise.

"No, no, my friend," he declared. "I do well to be careful. Do not forget the Big Four are on our track."

However, he soon elicited the information that a nurse of the name of Mabel Palmer was a member of the Lark Institute and had been sent to the case in question.

"So far, so good," he said, with a twinkle. "And now here comes Nurse Palmer back again, and here also is our analyst's report."

Both the nurse and I waited anxiously whilst Poirot read the analyst's report.

"Is there arsenic in it?" she asked breathlessly.

Poirot shook his head, refolding the paper.

"No."

We were both immeasurably surprised.

"There is no arsenic in it," continued Poirot. "But there is antimony. And that being the case, we will start immediately for Hertfordshire. Pray Heaven that we are not too late."

It was decided that the simplest plan was for Poirot to represent himself truly as a detective, but that the ostensible reason of his visit should be to question Mrs. Templeton about a servant formerly in her employment whose name he obtained from Nurse Palmer, and who he could represent as being concerned in a jewel robbery.

It was late when we arrived at Elmstead, as the house was called. We had allowed Nurse Palmer to precede us by about twenty minutes, so that there should be no question of our all arriving together.

Mrs. Templeton, a tall dark woman, with sinuous movements and uneasy eyes, received us. I noticed that as Poirot announced his profession, she drew in her breath with a sudden hiss, as though badly startled, but she answered his question about the maid-servant readily enough. And then, to test her, Poirot embarked upon a long history of a poisoning case in which a guilty wife had figured. His eyes never left her face as he talked, and try as she would, she could hardly conceal her rising agitation. Suddenly, with an incoherent word of excuse, she hurried from the room.

We were not long left alone. A squarely-built man with a small red moustache and pince-nez came in.

"Dr. Treves," he introduced himself. "Mrs. Templeton asked me to make her excuses to you. She's in a very bad state, you know. Nervous strain. Worry over her husband and all that. I've prescribed bed and bromide. But she hopes you'll stay and take pot luck, and I'm to do host. We've heard of you down here, M. Poirot, and we mean to make the most of you. Ah, here's Micky!"

A shambling young man entered the room. He had a very round face, and foolish-looking eyebrows raised as though in perpetual surprise. He grinned awkwardly as he shook hands. This was clearly the "wanting" son.

Presently we all went in to dinner. Dr. Treves left the room—to open some wine, I think—-and suddenly the boy's physiognomy underwent a startling change. He leant forward, staring at Poirot.

"You've come about father," he said, nodding his head. "I know. I know lots of things—but nobody thinks I do. Mother will be glad when father's dead and she can marry Dr. Treves. She isn't my own mother, you know. I don't like her. She wants father to die."

It was all rather horrible. Luckily, before Poirot had time to reply, the doctor came back, and we had to carry on a forced conversation.

And then suddenly Poirot lay back in his chair with a hollow groan. His face was contorted with pain.

"My dear sir, what's the matter?" cried the doctor.

"A sudden spasm. I am used to them. No, no, I require no assistance from you, doctor. If I might lie down upstairs."

His request was instantly acceded to, and I accompanied him upstairs, where he collapsed on the bed, groaning heavily.

For the first minute or two I had been taken in, but I had quickly realised that Poirot was—as he would have put it—playing the comedy, and that his object was to be left alone upstairs near the patient's room.

Hence I was quite prepared when, the instant we were alone, he sprang up.

"Quick, Hastings, the window. There is ivy outside. We can climb down before they begin to suspect."

"Climb down?"

"Yes, we must get out of this house at once. You saw him at dinner?"

"The doctor?"

"No, young Templeton. His trick with his bread. Do you remember what Flossie Monro told us before she died? That Claud Darrell had a habit of dabbing his bread on the table to pick up crumbs. Hastings, this is a vast plot, and that vacant-looking young man is our arch enemy—Number Four! Hurry."

I did not wait to argue. Incredible as the whole thing seemed, it was wiser not to delay. We scrambled down the ivy as quietly as we could and made a bee-line for the small town and the railway station. We were just able to catch the last train, the 8.34 which would land us in town about eleven o'clock.

"A plot," said Poirot thoughtfully. "How many of them were in it, I wonder? I suspect that the whole Templeton family are just so many agents of the Big Four. Did they simply want to decoy us down there? Or was it more subtle than that. Did they intend to play the comedy down there and keep me interested until they had had time to do—what? I wonder now."

He remained very thoughtful.

Arrived at our lodgings, he restrained me at the door of the sitting-room.

"Attention, Hastings. I have my suspicions. Let me enter first."

He did so, and, to my slight amusement, took the precaution to press on the electric switch with an old galosh. Then he went round the room like a strange cat, cautiously, delicately, on the alert for danger. I watched him for some time, remaining obediently where I had been put by the wall.

"It's all right, Poirot," I said impatiently.

"It seems so, mon ami, it seems so. But let us make sure."

"Rot," I said. "I shall light the fire, anyway, and have a pipe. I've caught you out for once. You had the matches last and you didn't put them back in the holder as usual—the very thing you're always cursing me for doing."

I stretched out my hand. I heard Poirot's warning cry—saw him leaping towards me—my hand touched the match-box.

Then—a flash of blue flame—an ear-rending crash—and darkness—

I came to myself to find the familiar face of our old friend Dr. Ridgeway bending over me. An expression of relief passed over his features.

"Keep still," he said soothingly. "You're all right. There's been an accident, you know."

"Poirot?" I murmured.

"You're in my digs. Everything's quite all right."

A cold fear clutched at my heart. His evasion woke a horrible fear.

"Poirot?" I reiterated. "What of Poirot."

He saw that I had to know and that further evasions were useless.

"By a miracle you escaped—Poirot—did not!"

A cry burst from my lips.

"Not dead? Not dead?"

Ridgeway bowed his head, his features working with emotion.

With desperate energy I pulled myself to a sitting position.

"Poirot may be dead," I said weakly. "But his spirit lives on. I will carry on his work! Death to the Big Four!"

Then I fell back, fainting.