"Yes," interrupted Polly eagerly, since, for once, her acumen had been at least as sharp as his, "but suspicion of that horrible crime only shifted its taint from one friend to another, and, of course, I know — "

"But that's just it," he quietly interrupted, "you don't know — Mr. Walter Hatherell, of course, you mean. So did every one else at once. The friend, weak and willing, committing a crime on behalf of his cowardly, yet more assertive friend who had tempted him to evil. It was a good theory; and was held pretty generally, I fancy, even by the police.

"I say 'even' because they worked really hard in order to build up a case against young Hatherell, but the great difficulty was that of time. At the hour when the policeman had seen the two men outside Park Square together, Walter Hatherell was still sitting in the Harewood Club, which he never left until twenty minutes to two. Had he wished to waylay and rob Aaron Cohen he would not have waited surely till the time when presumably the latter would already have reached home.

"Moreover, twenty minutes was an incredibly short time in which to walk from Hanover Square to Regent's Park without the chance of cutting across the squares, to look for a man, whose whereabouts you could not determine to within twenty yards or so, to have an argument with him, murder him, and ransack his pockets. And then there was the total absence of motive."

"But — " said Polly meditatively, for she remembered now that the Regent's Park murder, as it had been popularly called, was one of those which had remained as impenetrable a mystery as any other crime had ever been in the annals of the police.

The man in the corner cocked his funny birdlike head well on one side and looked at her, highly amused evidently at her perplexity.

"You do not see how that murder was committed?" he asked with a grin.

Polly was bound to admit that she did not.

"If you had happened to have been in Mr. John Ashley's predicament," he persisted, "you do not see how you could conveniently have done away with Mr. Aaron Cohen, pocketed his winnings, and then led the police of your country entirely by the nose, by proving an indisputable alibi?"

"I could not arrange conveniently," she retorted, "to be in two different places half a mile apart at one and the same time."

"No! I quite admit that you could not do this unless you also had a friend — "

"A friend? But you say — "

"I say that I admired Mr. John Ashley, for his was the head which planned the whole thing, but he could not have accomplished the fascinating and terrible drama without the help of willing and able hands."

"Even then — " she protested.

"Point number one," he began excitedly, fidgeting with his inevitable piece of string. "John Ashley and his friend Walter Hatherell leave the club together, and together decide on the plan of campaign. Hatherell returns to the club, and Ashley goes to fetch the revolver — the revolver which played such an important part in the drama, but not the part assigned to it by the police. Now try to follow Ashley closely, as he dogs Aaron Cohen's footsteps. Do you believe that he entered into conversation with him? That he walked by his side? That he asked for delay? No! He sneaked behind him and caught him by the throat, as the garroters used to do in the fog. Cohen was apoplectic, and Ashley is young and powerful. Moreover, he meant to kill — "

"But the two men talked together outside the Square gates," protested Polly, "one of whom was Cohen, and the other Ashley."

"Pardon me," he said, jumping up in his seat like a monkey on a stick, "there were not two men talking outside the Square gates. According to the testimony of James Funnell, the constable, two men were leaning arm in arm against the railings and one man was talking."

"Then you think that — "

"At the hour when James Funnell heard Holy Trinity clock striking half-past two Aaron Cohen was already dead. Look how simple the whole thing is," he added eagerly, "and how easy after that — easy, but oh, dear me! how wonderfully, how stupendously clever. As soon as James Funnell has passed on, John Ashley, having opened the gate, lifts the body of Aaron Cohen in his arms and carries him across the Square. The Square is deserted, of course, but the way is easy enough, and we must presume that Ashley had been in it before. Anyway, there was no fear of meeting any one.

"In the meantime Hatherell has left the club: as fast as his athletic legs can carry him he rushes along Oxford Street and Portland Place. It had been arranged between the two miscreants that the Square gate should be left on the latch.

"Close on Ashley's heels now, Hatherell too cuts across the Square, and reaches the further gate in good time to give his confederate a hand in disposing the body against the railings. Then, without another instant's delay, Ashley runs back across the gardens, straight to the Ashton Club, throwing away the keys of the dead man, on the very spot where he had made it a point of being seen and heard by a passer-by.

"Hatherell gives his friend six or seven minutes' start, then he begins the altercation which lasts two or three minutes, and finally rouses the neighbourhood with cries of 'Murder' and report of pistol in order to establish that the crime was committed at the hour when its perpetrator has already made out an indisputable alibi."

"I don't know what you think of it all, of course," added the funny creature as he fumbled for his coat and his gloves, "but I call the planning of that murder — on the part of novices, mind you — one of the cleverest pieces of strategy I have ever come across. It is one of those cases where there is no possibility whatever now of bringing the crime home to its perpetrator or his abettor. They have not left a single proof behind them; they foresaw everything, and each acted his part with a coolness and courage which, applied to a great and good cause, would have made fine statesmen of them both.

"As it is, I fear, they are just a pair of young blackguards, who have escaped human justice, and have only deserved the full and ungrudging admiration of yours very sincerely."

He had gone. Polly wanted to call him back, but his meagre person was no longer visible through the glass door. There were many things she would have wished to ask of him — what were his proofs, his facts? His were theories, after all, and yet, somehow, she felt that he had solved once again one of the darkest mysteries of great criminal London.