Poirot and I often dined at a small restaurant in Soho. We were there one evening, when we observed a friend at an adjacent table. It was Inspector Japp, and as there was room at our table, he came and joined us. It was some time since either of us had seen him.

"Never do you stop in to see us nowadays," declared Poirot reproachfully. "Not since the affair of the Yellow Jasmine have we met, and that is nearly a month ago."

"I've been up north—that's why. How are things with you? Big Four still going strong—eh?"

Poirot shook a finger at him reproachfully.

"Ah! you mock yourself at me—but the Big Four—they exist."

"Oh! I don't doubt that—but they're not the hub of the universe, as you make out."

"My friend, you are very much mistaken. The greatest power for evil in the world to-day is this 'Big Four.' To what end they are tending, no one knows, but there has never been another such criminal organisation. The finest brain in China at the head of it, an American millionaire, and a French woman scientist as members, and for the fourth—"

Japp interrupted.

"I know—I know. Regular bee in your bonnet over it all. It's becoming your little mania, Moosior Poirot. Let's talk of something else for a change. Take any interest in chess?"

"I have played it, yes."

"Did you see that curious business yesterday? Match between two players of world-wide reputation, and one died during the game?"

"I saw a mention of it. Dr. Savaronoff, the Russian champion, was one of the players, and the other, who succumbed to heart failure, was the brilliant young American, Gilmour Wilson."

"Quite right. Savaronoff beat Rubenstein and became Russian champion some years ago. Wilson is said to be a second Capablanca."

"A very curious occurrence," mused Poirot. "If I mistake not, you have a particular interest in the matter?"

Japp gave a rather embarrassed laugh.

"You've hit it, Moosior Poirot. I'm puzzled. Wilson was sound as a bell—no trace of heart trouble. His death is quite inexplicable."

"You suspect Dr. Savaronoff of putting him out of the way?" I cried.

"Hardly that," said Japp dryly. "I don't think even a Russian would murder another man in order not to be beaten at chess—and anyway, from all I can make out, the boot was likely to be on the other leg. The doctor is supposed to be very hot stuff—second to Lasker they say he is."

Poirot nodded thoughtfully.

"Then what exactly is your little idea?" he asked. "Why should Wilson be poisoned? For, I assume, of course, that it is poison you suspect."

"Naturally. Heart failure means your heart stops beating—that's all there is to that. That's what a doctor says officially at the moment, but privately he tips us the wink that he's not satisfied."

"When is the autopsy to take place?"

"To-night. Wilson's death was extraordinarily sudden. He seemed quite as usual and was actually moving one of the pieces when he suddenly fell forward—dead!"

"There are very few poisons would act in such a fashion," objected Poirot.

"I know. The autopsy will help us, I expect. But why should any one want Gilmour Wilson out of the way—that's what I'd like to know? Harmless unassuming young fellow. Just come over here from the States, and apparently hadn't an enemy in the world."

"It seems incredible," I mused.

"Not at all," said Poirot, smiling. "Japp has his theory, I can see."

"I have, Moosior Poirot. I don't believe the poison was meant for Wilson—it was meant for the other man."

"Savaronoff?"

"Yes. Savaronoff fell foul of the Bolsheviks at the outbreak of the Revolution. He was even reported killed. In reality he escaped, and for three years endured incredible hardships in the wilds of Siberia. His sufferings were so great that he is now a changed man. His friends and acquaintances declare they would hardly have recognised him. His hair is white, and his whole aspect that of a man terribly aged. He is a semi-invalid, and seldom goes out, living alone with a niece, Sonia Daviloff, and a Russian man-servant in a flat down Westminster way. It is possible that he still considers himself a marked man. Certainly he was very unwilling to agree to this chess contest. He refused several times point blank, and it was only when the newspapers took it up and began making a fuss about the 'unsportsmanlike refusal' that he gave in. Gilmour Wilson had gone on challenging him with real Yankee pertinacity, and in the end he got his way. Now I ask you, Moosior Poirot, why wasn't he willing? Because he didn't want attention drawn to him. Didn't want somebody or other to get on his track. That's my solution—Gilmour Wilson got pipped by mistake."

"There is no one who has any private reason to gain by Savaronoff's death?"

"Well, his niece, I suppose. He's recently come into an immense fortune. Left him by Madame Gospoja whose husband was a sugar profiteer under the old regime. They had an affair together once, I believe, and she refused steadfastly to credit the reports of his death."

"Where did the match take place?"

"In Savaronoff's own flat. He's an invalid, as I told you."

"Many people there to watch it?"

"At least a dozen—probably more."

Poirot made an expressive grimace.

"My poor Japp, your task is not an easy one."

"Once I know definitely that Wilson was poisoned, I can get on."

"Has it occurred to you that, in the meantime, supposing your assumption that Savaronoff was the intended victim to be correct, the murderer may try again?"

"Of course it has. Two men are watching Savaronoff's flat."

"That will be very useful if any one should call with a bomb under his arm," said Poirot dryly.

"You're getting interested, Moosior Poirot," said Japp, with a twinkle. "Care to come round to the mortuary and see Wilson's body before the doctors start on it? Who knows, his tie-pin may be askew, and that may give you a valuable clue that will solve the mystery."

"My dear Japp, all through dinner my fingers have been itching to rearrange your own tie-pin. You permit, yes? Ah! that is much more pleasing to the eye. Yes, by all means, let us go to the mortuary."

I could see that Poirot's attention was completely captivated by this new problem. It was so long since he had shown any interest over any outside case that I was quite rejoiced to see him back in his old form.

For my own part, I felt a deep pity as I looked down upon the motionless form and convulsed face of the hapless young American who had come by his death in such a strange way. Poirot examined the body attentively. There was no mark on it anywhere, except a small scar on the left hand.

"And the doctor says that's a burn, not a cut," explained Japp.

Poirot's attention shifted to the contents of the dead man's pockets which a constable spread out for our inspection. There was nothing much—a handkerchief, keys, notecase filled with notes, and some unimportant letters. But one object standing by itself filled Poirot with interest.

"A chessman!" he exclaimed. "A white bishop. Was that in his pocket?"

"No, clasped in his hand. We had quite a difficulty to get it out of his fingers. It must be returned to Dr. Savaronoff sometime. It's part of a very beautiful set of carved ivory chessmen."

"Permit me to return it to him. It will make an excuse for my going there."

"Aha!" cried Japp. "So you want to come in on this case?"

"I admit it. So skilfully have you aroused my interest."

"That's fine. Got you away from your brooding. Captain Hastings is pleased, too, I can see."

"Quite right," I said, laughing.

Poirot turned back towards the body.

"No other little detail you can tell me about—him?" he asked.

"I don't think so."

"Not even—that he was left-handed?"

"You're a wizard, Moosior Poirot. How did you know that? He was left-handed. Not that it's anything to do with the case."

"Nothing whatever," agreed Poirot hastily, seeing that Japp was slightly ruffled. "My little joke—that was all. I like to play you the trick, see you."

We went out upon an amicable understanding.

The following morning saw us wending our way to Dr. Savaronoff's flat in Westminster.

"Sonia Daviloff," I mused. "It's a pretty name."

Poirot stopped, and threw me a look of despair.

"Always looking for romance! You are incorrigible. It would serve you right if Sonia Daviloff turned out to be our friend and enemy the Countess Vera Rossakoff."

At the mention of the countess, my face clouded over.

"Surely, Poirot, you don't suspect—"

"But, no, no. It was a joke! I have not the Big Four on the brain to that extent, whatever Japp may say."

The door of the flat was opened to us by a man-servant with a peculiarly wooden face. It seemed impossible to believe that that impassive countenance could ever display emotion.

Poirot presented a card on which Japp had scribbled a few words of introduction, and we were shown into a low, long room furnished with rich hangings and curios. One or two wonderful ikons hung upon the walls, and exquisite Persian rugs lay upon the floor. A samovar stood upon a table.

I was examining one of the ikons which I judged to be of considerable value, and turned to see Poirot prone upon the floor. Beautiful as the rug was, it hardly seemed to me to necessitate such close attention.

"Is it such a very wonderful specimen?" I asked.

"Eh? Oh! the rug? But no, it was not the rug I was remarking. But it is a beautiful specimen, far too beautiful to have a large nail wantonly driven through the middle of it. No, Hastings," as I came forward, "the nail is not there now. But the hole remains."

A sudden sound behind us made me spin round, and Poirot spring nimbly to his feet. A girl was standing in the doorway. Her eyes, full upon us, were dark with suspicion. She was of medium height, with a beautiful, rather sullen face, dark blue eyes, and very black hair which was cut short. Her voice, when she spoke, was rich and sonorous, and completely un-English.

"I fear my uncle will be unable to see you. He is a great invalid."

"That is a pity, but perhaps you will kindly help me instead. You are Mademoiselle Daviloff, are you not?"

"Yes, I am Sonia Daviloff. What is it you want to know?"

"I am making some inquiries about that sad affair the night before last—the death of M. Gilmour Wilson. What can you tell me about it?"

The girl's eyes opened wide.

"He died of heart failure—as he was playing chess."

"The police are not so sure that it was—heart failure, mademoiselle."

The girl gave a terrified gesture.

"It was true then," she cried. "Ivan was right."

"Who is Ivan, and why do you say he was right?"

"It was Ivan who opened the door to you—and he has already said to me that in his opinion Gilmour Wilson did not die a natural death—that he was poisoned by mistake."

"By mistake."

"Yes, the poison was meant for my uncle."

She had quite forgotten her first distrust now, and was speaking eagerly.

"Why do you say that, mademoiselle. Who should wish to poison Dr. Savaronoff?"

She shook her head.

"I do not know. I am all in the dark. And my uncle, he will not trust me. It is natural, perhaps. You see, he hardly knows me. He saw me as a child, and not since till I came to live with him here in London. But this much I do know, he is in fear of something. We have many secret societies in Russia, and one day I overheard something which made me think it was of just such a society he went in fear. Tell me, monsieur"—she came a step nearer, and dropped her voice—"have you ever heard of a society called the 'Big Four'?"

Poirot jumped nearly out of his skin. His eyes positively bulged with astonishment.

"Why do you—what do you know of the Big Four, mademoiselle?"

"There is such an association, then! I overheard a reference to them, and asked my uncle about it afterwards. Never have I seen a man so afraid. He turned all white and shaking. He was in fear of them, monsieur, in great fear, I am sure of it. And, by mistake, they killed the American, Wilson."

"The Big Four," murmured Poirot. "Always the Big Four! An astonishing coincidence, mademoiselle, your uncle is still in danger. I must save him. Now recount to me exactly the events of that fatal evening. Show me the chess-board, the table, how the two men sat—everything."

She went to the side of the room and brought out a small table. The top of it was exquisite, inlaid with squares of silver and black to represent a chess-board.

"This was sent to my uncle a few weeks ago as a present, with the request that he would use it in the next match he played. It was in the middle of the room—so."

Poirot examined the table with what seemed to me quite unnecessary attention. He was not conducting the inquiry at all as I would have done. Many of his questions seemed to me pointless, and upon really vital matters he seemed to have no questions to ask. I concluded that the unexpected mention of the Big Four had thrown him completely off his balance.

After a minute examination of the table and the exact position it had occupied, he asked to see the chessmen. Sonia Daviloff brought them to him in a box. He examined one or two of them in a perfunctory manner.

"An exquisite set," he murmured absent-mindedly.

Still not a question as to what refreshments there had been, or what people had been present.

I cleared my throat significantly.

"Don't you think, Poirot, that—"

He interrupted me peremptorily.

"Do not think, my friend. Leave all to me. Mademoiselle, is it quite impossible that I should see your uncle?"

A faint smile showed itself on her face.

"He will see you, yes. You understand, it is my part to interview all strangers first."

She disappeared. I heard a murmur of voices in the next room, and a minute later she came back and motioned us to pass into the adjoining room.

The man who lay there on a couch was an imposing figure. Tall, gaunt, with huge bushy eyebrows and white beard, and a face haggard as the result of starvation and hardships. Dr. Savaronoff was a distinct personality. I noted the peculiar formation of his head, its unusual height. A great chess player must have a great brain, I knew. I could easily understand Dr. Savaronoff being the second greatest player in the world.

Poirot bowed.

"M. le Docteur, may I speak to you alone?"

Savaronoff turned to his niece.

"Leave us, Sonia."

She disappeared obediently.

"Now, sir, what is it?"

"Dr. Savaronoff, you have recently come into an enormous fortune. If you should—die unexpectedly, who inherits it?"

"I have made a will leaving everything to my niece, Sonia Daviloff. You do not suggest—"

"I suggest nothing, but you have not seen your niece since she was a child. It would have been easy for any one to impersonate her."

Savaronoff seemed thunderstruck by the suggestion. Poirot went on easily.

"Enough as to that. I give you the word of warning, that is all. What I want you to do now is to describe to me the game of chess the other evening."

"How do you mean—describe it?"

"Well, I do not play the chess myself, but I understand that there are various regular ways of beginning—the gambit, do they not call it?"

Dr. Savaronoff smiled a little.

"Ah! I comprehend you now. Wilson opened Ruy Lopez—one of the soundest openings there is, and one frequently adopted in tournaments and matches."

"And how long had you been playing when the tragedy happened?"

"It must have been about the third or fourth move when Wilson suddenly fell forward over the table, stone dead."

Poirot rose to depart. He flung out his last question as though it was of absolutely no importance, but I knew better.

"Had he had anything to eat or drink?"

"A whisky and soda, I think."

"Thank you, Dr. Savaronoff. I will disturb you no longer."

Ivan was in the hall to show us out. Poirot lingered on the threshold.

"The flat below this, do you know who lives there?"

"Sir Charles Kingwell, a member of Parliament, sir. It has been let furnished lately, though."

"Thank you."

We went out into the bright winter sunlight.

"Well, really, Poirot," I burst out. "I don't think you've distinguished yourself this time. Surely your questions were very inadequate."

"You think so, Hastings?" Poirot looked at me appealingly. "I was bouleversé, yes. What would you have asked?"

I considered the question carefully, and then outlined my scheme to Poirot. He listened with what seemed to be close interest. My monologue lasted until we had nearly reached home.

"Very excellent, very searching, Hastings," said Poirot, as he inserted his key in the door and preceded me up the stairs. "But quite unnecessary."

"Unnecessary!" I cried, amazed. "If the man was poisoned—"

"Aha," cried Poirot, pouncing upon a note which lay on the table. "From Japp. Just as I thought." He flung it over to me. It was brief and to the point. No traces of poison had been found, and there was nothing to show how the man came by his death.

"You see," said Poirot, "our questions would have been quite unnecessary."

"You guessed this beforehand?"

"'Forecast the probable result of the deal,'" quoted Poirot from a recent Bridge problem on which I had spent much time. "Mon ami, when you do that successfully, you do not call it guessing."

"Don't let's split hairs," I said impatiently. "You foresaw this?"

"I did."

"Why?"

Poirot put his hand into his pocket and pulled out—a white bishop.

"Why," I cried, "you forgot to give it back to Dr. Savaronoff."

"You are in error, my friend. That bishop still reposes in my left-hand pocket. I took its fellow from the box of chessmen Mademoiselle Daviloff kindly permitted me to examine. The plural of one bishop is two bishops."

He sounded the final "s" with a great hiss. I was completely mystified.

"But why did you take it?"

"Parbleu, I wanted to see if they were exactly alike."

He stood them on the table side by side.

"Well, they are, of course," I said, "exactly alike."

Poirot looked at them with his head on one side.

"They seem so, I admit. But one should take no fact for granted until it is proved. Bring me, I pray you, my little scales."

With infinite care he weighed the two chessmen, then turned to me with a face alight with triumph.

"I was right. See you, I was right. Impossible to deceive Hercule Poirot!"

He rushed to the telephone—waited impatiently.

"Is that Japp? Ah! Japp, it is you. Hercule Poirot speaks. Watch the man-servant, Ivan. On no account let him slip through your fingers. Yes, yes, it is as I say."

He dashed down the receiver and turned to me.

"You see it not, Hastings? I will explain. Wilson was not poisoned, he was electrocuted. A thin metal rod passes up the middle of one of those chessmen. The table was prepared beforehand and set upon a certain spot on the floor. When the bishop was placed upon one of the silver squares, the current passed through Wilson's body, killing him instantly. The only mark was the electric burn upon his hand—his left hand, because he was left-handed. The 'special table' was an extremely cunning piece of mechanism. The table I examined was a duplicate, perfectly innocent. It was substituted for the other immediately after the murder. The thing was worked from the flat below, which, if you remember, was let furnished. But one accomplice at least was in Savaronoff's flat. The girl is an agent of the Big Four, working to inherit Savaronoff's money."

"And Ivan?"

"I strongly suspect that Ivan is none other than the famous Number Four."

"What?"

"Yes. The man is a marvellous character actor. He can assume any part he pleases."

I thought back over past adventures, the lunatic asylum keeper, the butcher's young man, the suave doctor, all the same man, and all totally unlike each other.

"It's amazing," I said at last. "Everything fits in. Savaronoff had an inkling of the plot, and that's why he was so averse to playing the match."

Poirot looked at me without speaking. Then he turned abruptly away, and began pacing up and down.

"Have you a book on chess by any chance, mon ami?" he asked suddenly.

"I believe I have somewhere."

It took me some time to ferret it out, but I found it at last, and brought it to Poirot, who sank down in a chair and started reading it with the greatest attention.

In about a quarter of an hour the telephone rang. I answered it. It was Japp. Ivan had left the flat, carrying a large bundle. He had sprung into a waiting taxi, and the chase had begun. He was evidently trying to lose his pursuers. In the end he seemed to fancy that he had done so, and had then driven to a big empty house at Hampstead. The house was surrounded.

I recounted all this to Poirot. He merely stared at me as though he scarcely took in what I was saying. He held out the chess book.

"Listen to this, my friend. This is the Ruy Lopez Opening. 1 P-K4, P-K4; 2 Kt-KB3, Kt-QB3; 3B-Kt5;? Then there comes a question as to Black's best third move. He has the choice of various defences. It was White's third move that killed Gilmour Wilson, 3B-Kt5. Only the third move—does that say nothing to you?"

I hadn't the least idea what he meant, and told him so.

"I suppose, Hastings, that while you were sitting in this chair, you heard the front door being opened and shut, what would you think?"

"I should think some one had gone out, I suppose."

"Yes—but there are always two ways of looking at things. Some one gone out—some one come in—two totally different things, Hastings. But if you assumed the wrong one, presently some little discrepancy would creep in and show you that you were on the wrong track."

"What does all this mean, Poirot?"

Poirot sprang to his feet with sudden energy.

"It means that I have been a triple imbecile. Quick, quick, to the flat in Westminster. We may yet be in time."

We tore off in a taxi. Poirot returned no answer to my excited questions. We raced up the stairs. Repeated rings and knocks brought no reply, but listening closely I could distinguish a hollow groan coming from within.

The hall porter proved to have a master key, and after a few difficulties he consented to use it.

Poirot went straight to the inner room. A whiff of chloroform met us. On the floor was Sonia Daviloff, gagged and bound, with a great wad of saturated cotton wool over her nose and mouth. Poirot tore it off and began to take measures to restore her. Presently a doctor arrived, and Poirot handed her over to his charge and drew aside with me. There was no sign of Dr. Savaronoff.

"What does it all mean?" I asked, bewildered.

"It means that before two equal deductions I chose the wrong one. You heard me say that it would be easy for any one to impersonate Sonia Daviloff because her uncle had not seen her for so many years?"

"Yes?"

"Well, precisely the opposite held good also. It was equally easy for any one to impersonate the uncle."

"What?"

"Savaronoff did die at the outbreak of the Revolution. The man who pretended to have escaped with such terrible hardships, the man so changed 'that his own friends could hardly recognise him,' the man who successfully laid claim to an enormous fortune—"

"Yes. Who was he?"

"Number Four. No wonder he was frightened when Sonia let him know she had overheard one of his private conversations about the 'Big Four.' Again he has slipped through my fingers. He guessed I should get on the right track in the end, so he sent off the honest Ivan on a tortuous wild goose chase, chloroformed the girl, and got out, having by now doubtless realised most of the securities left by Madame Gospoja."

"But—but who tried to kill him then?"

"Nobody tried to kill him. Wilson was the intended victim all along."

"But why?"

"My friend, Savaronoff was the second greatest chess player in the world. In all probability Number Four did not even know the rudiments of the game. Certainly he could not sustain the fiction of a match. He tried all he knew to avoid the contest. When that failed, Wilson's doom was sealed. At all costs he must be prevented from discovering that the great Savaronoff did not even know how to play chess. Wilson was fond of the Ruy Lopez opening, and was certain to use it. Number Four arranged for death to come with the third move, before any complications of defence set in."

"But, my dear Poirot," I persisted, "are we dealing with a lunatic? I quite follow your reasoning, and admit that you must be right, but to kill a man just to sustain his r?le! Surely there were simpler ways out of the difficulty than that? He could have said that his doctor forbade the strain of a match."

Poirot wrinkled his forehead.

"Certainement, Hastings," he said, "there were other ways, but none so convincing. Besides, you are assuming that to kill a man is a thing to avoid, are you not? Number Four's mind, it does not act that way. I put myself in his place, a thing impossible for you. I picture his thoughts. He enjoys himself as the professor at that match. I doubt not he has visited the chess tourneys to study his part. He sits and frowns in thought; he gives the impression that he is thinking great plans, and all the time he laughs in himself. He is aware that two moves are all that he knows—and all that he need know. Again, it would appeal to his mind to foresee the events and to make the man his own executioner at the exact time that suits Number Four.... Oh, yes, Hastings, I begin to understand our friend and his psychology."

I shrugged.

"Well, I suppose you're right, but I can't understand any one running a risk he could so easily avoid."

"Risk!" Poirot snorted. "Where then lay the risk? Would Japp have solved the problem? No; if Number Four had not made one small mistake he would have run no risk."

"And his mistake?" I asked, although I suspected the answer.

"Mon ami, he overlooked the little gray cells of Hercule Poirot."

Poirot has his virtues, but modesty is not one of them.