It was the most extraordinary thing in the world. Beyond question the room was absolutely empty. Jack could see to the far side; he noted the pictures and the flowers and the vases on the mantelpiece. His view was naturally narrowed by a small spyhole, but there was no portion of the room hidden from him, though he could not quite see the whole of it at one time.

The music was proceeding quite smoothly, though with pauses now and again. It was followed now and then by what sounded like subdued applause.

Jack stepped back from the window. He wanted to make certain that he had not mistaken the room. No, the sounds of music came from the study right enough. At the risk of being discovered he crept back into the house again and tried the study door. It was locked, and what was more, the key was in the lock, as the application of an eye testified.

And the music was proceeding quite swiftly again. The mystery was absolutely maddening. Jack wondered if there was some cabinet in the study hidden from view where the player had taken up his stand. At any rate somebody was playing Chopin's music--playing it very well. There was no magic about the thing.

The hall of the house was very quiet, nobody seemed to be about. Occasionally there came the sound of mirth from the servants' hall, but nothing more. Fully determined to get to the bottom of this mystery, Jack returned to the garden again. Once more his eye was glued to the slit in the blind. He could make nobody out in the room. There was little fear of his being detected, because a belt of shrubs hid the window from the road.

Without the slightest warning a figure appeared in the room. It was impossible to see where she came from, but of necessity she must have entered by the door. Jack was a little uncertain on that head, for his glance was not directed towards the door for the moment.

He saw the figure of a woman, young and exceedingly well dressed. She was wearing an evening gown of white satin that showed up the creamy pallor of her skin, for her neck and shoulders were bare. The neck was rather thin, Jack noted, and the shoulders more inclined to muscle than beauty. For a young girl it struck Jack that the upper part of her body looked old. But the face was dark and wholesome, and against the deep eyes and swarthy complexion the girl's hair was dazzling. It was beautiful, rippling hair, changing color as the light flashed upon it.

"Well, this is a bit of an adventure," the watcher told himself. "But where's the person in the room who let the young lady in? Somebody must have let her in, because the door was locked and the key on the inside. I saw it there, so I can swear to that fact. But who is she?"

There were many answers to the problem, for Spencer Anstruther was a man who had countless strange visitors. His vast knowledge of crime and the ramifications of human depravity brought him in contact with large numbers of people. Men and women in distress often came to him, and they came in increasing numbers since Anstruther had got the better of a gang of scoundrels in a recent famous blackmailing case. Sometimes these people came on their own initiative, sometimes they were sent by the police. But Anstruther never said anything about them. He looked upon himself as a confidential agent. Claire could have told of many curious visitors at all hours, though Anstruther never so much as alluded to them afterwards.

But this girl did not look in the least like anybody in trouble. Her dark features were almost expressionless; there was no display of violent emotions there. Her gaze slowly wandered round the room as if looking for something; she had much the aspect of a pupil whose attention is called to a blackboard by a master. As Jack watched, it seemed to him that he had seen this girl before. He could not recollect anybody in the least like her; that contrast of dark skin and fair hair was striking enough to impress itself upon the most careless mind, and yet Jack could not give the face a name. He could not permit himself to believe that he had made a mistake. He knew perfectly well that the expressionless features were quite familiar to him.

The girl stood for some little time, as if waiting for her lesson. Jack's eyes were glued so closely upon her that he did not notice the coming of another person--a man this time. He was a young man, with sleek, well-brushed brown hair, and dark, well-groomed moustache turned up after the fashion affected by the German Emperor. The man was perfectly well appointed, his evening dress and white waistcoat were faultless. His face was strong, but it did not convey anything intellectual. There were scores of such men to be seen any day during the London season, all groomed the same, all apparently finished in the same machine.

The man bowed and smiled to the lady, and she bowed and smiled in return. It was rather a graceful bow; it seemed to Jack that she looked at her companion to see if it were quite correct. Then the two proceeded to talk in dumb show, partly by signs and partly by fingers. The mystery was getting deeper--one of these two was a deaf mute, perhaps both of them. Was this one of Anstruther's cases, or did it possess a far deeper significance?

The solution was beyond Jack Masefield. He might have been on the track of a mystery, and on the other hand he might merely be doing a little vulgar eavesdropping. If it was the latter, and Anstruther found him out, he need not hope to visit Claire at home any more. Anstruther was most particular about these things, as Jack knew; but he set his teeth together and decided to take the risk. He felt pretty sure that there was something here that touched the household deeply.

He turned just for the moment, with an idea that somebody was behind him. But the strip of lawn was quite clear. Jack could see through the belt of trees to the street again beyond, with its great arc light flaring on the yellow face of the mysterious Nostalgo and his starting, half-laughing eyes. That weird face seemed to form a fitting background to the room mystery.

But Jack had his eyes to the slit in the blind again. Inside the pantomime in show was still going on. The girl seemed to be getting a lesson of some kind, and her tutor appeared to be pleased, for he smiled and clapped his hands from time to time. Then he took out his watch and consulted it with a frown. As he glanced up the girl crossed the room to the mantelpiece and opened the face of the clock. With a quick movement she put it back half-an-hour.

The man in the faultless evening dress nodded approval. There was a little pause before he approached the window and stood so that his shadow was picked out clean against the strong light of the room. Then he rapidly signaled with his arm. One arm went up, there was a noise of rings and a flutter of drapery, and then a heavy curtain was jerked over the window, and Jack could see no more. Try as he would, no ray of light could he make out. It was as if the lights had been switched off, leaving the room in utter darkness.

What on earth did it all mean? Beyond doubt the young man in evening dress had signaled to somebody outside when he stood close against the window and raised his arm. Jack congratulated himself on the fact that the slit in the blind was low down, so that he had not to stand against the light. He slipped into the belt of shrubs and watched for a moment, but no further sign came.

What were those people inside going to do? The solution flashed upon Jack instantly. They had not come there so perfectly dressed for the mere sake of seeing Spencer Anstruther. They had not been spending the evening anywhere, dining and that kind of thing beforehand, for they looked too spruce and fresh for that. The woman's toilette in particular had evidently been just donned, as if fresh from the hands of her maid. And she had put the clock back half-an-hour.

"They are going somewhere in half-an-hour," Jack decided. "Hang me if I don't follow them. By the right time it is half-past ten. Anstruther said he should not come up if he failed to get his business finished before eleven, at which time he will expect me to go. I'll go up to the drawing-room and talk to Claire for a little time just to avert suspicion."

He crept back into the house without being seen, he finished his claret, and dropped the stump of his cigarette on to his dessert plate. As he made his way up the stairs the music began again. That music was not the least maddening part of the mystery.

"What a time you have been," Claire said as she tossed her book aside. "All by yourself down there! Really, Jack, you modern young men are so cold-blooded that----"

"I'm not so far as you are concerned, dearest," Jack, said as he kissed the girl. "I had something to do; I was working out a case that puzzled me."

"A case in some way connected with the law, I suppose?" Claire asked.

"Well, yes," Jack replied. He quite believed that the case was connected with the law. "I begin to see my way to its solution. I suppose there is not the slightest chance of your guardian coming up to-night?"

Claire replied that it did not look like it. Evidently the solution of the music problem was not an easy one, for the violin was going again as if it had only just begun.

"It makes me feel creepy," Claire exclaimed. "Fancy the idea of tracking a criminal by means of divine melody like that! Jack, don't you notice something strange about it?"

"I should say that I do," Jack said. "Why, the whole thing--really, I beg your pardon, darling. I--I was thinking about something else. It was the case I alluded to just now."

"My dear boy, you are very strange in your manner to-night," Claire said. "You look pale and distracted. Trust the eyes of love to see anything like that. You haven't bad news for me, Jack?"

Masefield forced a smile to his lips. It was hard work to maintain his ordinary manner in the face of the strange scene that he had witnessed that night.

"I have certainly heard no news since dinner time," he said. "What did you expect me to say?"

"I thought that perhaps you had mentioned me to my guardian; that you had changed your mind, and told him that you and I were going to be married some time."

"No, your name was never mentioned, dearest. Anstruther was full of his case and gave me no opportunity. He went off directly he had finished his tobacco. As a matter of fact, Claire, I am more resolved than ever to say nothing about our engagement to Mr. Anstruther."

"It is very strange that you mistrust him like that, Jack."

"Perhaps it is, little woman. Call it instinct, if you like. I know that women are supposed to hold the monopoly of that illogical faculty. They dislike a man or a woman without being able to say why, and in the course of time that man or woman turns out to be a villain. There is no denying the fact that I feel the same way towards your guardian. I am convinced that once he knows the truth you will be in danger. I said before that he is a poor man, and the enjoyment of your ￡2,000 during the time----"

"My dear Jack, you are perfectly horrid," Claire murmured. "If I were a nervous girl you would frighten me. As it is, I feel certain that you are utterly wrong. My guardian is one of the most delightful of men. If he were not, plenty of clever people would have found it out. And, besides, why do so many unfortunate people come to him to advise them, which he does with great trouble to himself and no hope of reward?"

Jack admitted that perhaps he was wrong. And he had no desire either to frighten Claire. He had not the slightest intention of telling her what he had discovered that night.

"Let us be less personal," he said. "What was the strange thing that you noticed about your guardian's playing?"

"That it is so much better than usual," Claire said. "There seemed more passion and feeling in the music. My guardian is a brilliant violin player, but I have not hitherto noticed much feeling in his style. Now, listen to the thing that he is playing at present."

"Chopin's Fantasie in F," Jack muttered. "I know it very well indeed. It is a favorite of mine."

There was certainly plenty of expression and feeling in the music. Jack was bound to admit that. The fantasie came to an end with a crash of two chords, and Claire clapped her hands.

"Beautiful!" she cried. "I must really compliment my guardian on the improvement in his style. You are not going already, Jack? It's not quite eleven yet."

"I'm very sorry, dear, but I have that case to look into to-night," Jack said, with perfect truth. He saw that the hands of the big clock on the mantelpiece were creeping on to the hour. "Anstruther won't come up to-night; he said he should be here by eleven if he were. And he gave me a hint not to stay later. I shall see you at the Warings' to-morrow night. Good-night, darling."

Claire put up her red lips to be kissed. She would have seen Jack to the door, but he pointed out that the night was chilly and Claire's dress thin. Neither would he have the butler summoned. His coat and hat were in the hall, and he would get them himself. A moment or two later and he was standing in the garden behind the strip of shrubs. He was quite free to act now; he had nobody in the way. As he stood there, a distant church clock boomed the hour of eleven.

"Now we shall see what we shall see," Jack muttered. "I'm going to find whether there is a mystery of the house or whether these people are merely Anstruther's clients. Oh!"

As he spoke the dark curtain over the study window was pulled back, and the figure of the young man in the evening dress was clean cut against the light. Then a black arm pulled for the catch of the window, and the young man, pushing the blind aside, came out. He was wearing an overcoat now, and a tall hat. He seemed to be waiting for somebody.

Then the figure of the dark-faced, fair-haired girl came out. She was cloaked from head to foot in a blue wrap trimmed with feathers; her fair hair was not covered. No word was spoken, but Jack could see that they were conversing still by signs.

The watcher wondered if he had time to get inside the room. But that little idea was dismissed at the outset, for the young man pushed the window to carefully and the latch clicked. It was quite evident that the long sash closed with a spring lock, which was a most unusual thing for French windows to do. As the strange pair went down the side path Jack stepped into the open. He wanted to assure himself as to the window being fastened. He pulled at it hard, but it did not yield. At the same moment from the window of the room came a strange, brilliant crash of music. Yet that room was absolutely empty, as Jack would have been prepared to swear in any court of England.

"I'll wake up either from a dream or in a lunatic asylum presently," he muttered. "And now for those other people. Good thing they had no idea of being followed."

Jack was in the road now, and taking his way through the quiet nest of squares between Bloomsbury and Regent's Park. He could see his quarry a hundred yards or so before him; there was nobody else, and there was not the slightest chance of those in front being lost. A horse's hoof clicked on the wood pavement as a well-appointed hansom passed the tracker. Then he saw the hansom pull up by the curb and the deaf mutes in front jump in, as if the whole thing had been arranged, and drive off.

The thing was so sudden and unexpected that Jack was nonplused for a moment. There was no chance of following these people, for there probably was not another hansom within half-a-mile of the spot. Jack stood hesitating in the silence of the road; he could hear the steady flick-flack of the horse's hoofs as the rubber-tired hansom hurried on, and then suddenly the horse's hoofs stopped. They had not died out in the distance; they had merely stopped.

Jack hurried forward; he had not given up all hope yet. He might overtake the hansom and by good luck meet an empty one going towards the Strand. As he turned a corner, he saw to his surprise the figure of the young man in evening dress come silently towards him on the other side of the road. Then the stranger crossed the road and turned down the far side of the square as if he were going to complete the circuit and join his cab again. As the man vanished Jack heard a thudding sound, followed by a sound like the tearing of stiff paper, like the rattle of peas on a drum, a queer stifled cry, and then silence. On the impulse of the moment, Jack turned and followed.

At the angle stood a row of houses, some of them being repaired. Jack heard somebody speak to somebody else a little way down the road. He looked across at the opposite houses to see that they were in scaffolding and that they were plastered with bills. A little way above the ground in front of the centre house being repaired was one of the repulsive, clever Nostalgo posters with the yellow face looking out.

But there was something else lying there at full length on the pavement, the body of a man with his face up to the stars. With a little cry Jack crossed the road. Almost instantly a policeman stood by his side.

"Drunk," he said. "A gentleman who's just gone down the road told me a man was lying drunk on the pavement. My word, sir, but he's got the complaint pretty bad."

"He has," Jack said, with a catch in his voice. "The man isn't drunk; he's dead. He's been murdered. Shot through the head and breast. Show your lantern here, officer."

The officer flashed the strong, searching rays on the face of the dead man. As he did so he gave a cry, and pointed to the hoarding behind him with a finger that shook a little.

"Dead, sir, and murdered, beyond doubt," he said. "But that's not the strangest part of it. Look at his face and the expression of his eyes; look at the yellow face and----"

"Good heaven!" Jack cried. "The yellow face, the face of the diabolical poster behind you. As I am a living man, we have found Nostalgo in the flesh."

The dead man grinned up, the poster grinned down. And the face of the dead and the face in the print were exactly the same!