Nick Carter wore a worried look at eight o’clock that evening. Both he and Chick then were dressing for the elaborate reception and ball tendered to the local National Guards, generally admitted to be the chief social event slated for that season in Madison, and during which the unknown crook whom the detectives were so anxious to identify had threatened to commit the crime the latter were grimly determined to prevent.

Nick Carter’s anxiety, however, was not because his life also had been threatened and might possibly be taken, in case he became an insurmountable obstacle to the designs of the mysterious and daring desperado. He was thinking of Patsy Garvan, his prolonged absence, the occasion for which he could not fathom, knowing that Patsy ordinarily would have reported by telephone, at least, in view of the work engaging him, unless something very unexpected and equally serious prevented him.

The detective did not blind himself, moreover, to the fact that his own designs had been repeatedly anticipated and balked by the unknown knave or by members of his gang, in spite of his own expeditious work and the precautions he had taken. He realized most keenly that he was up against a remarkably crafty and resourceful scoundrel. He began to fear that Patsy had fallen into his hands and, in spite of his confidence in his own skill and prowess, that he also might be booked for failure and utterly unable to prevent the threatened theft of Mrs. Mortimer Thurlow’s pearls.

“It would be perfectly easy to foil the rascals, if that was all we wished to accomplish,” said the detective, while he and Chick were discussing their plans. “But that is not enough.”

“Certainly not,” declared Chick. “We must take advantage of the circumstances to discover their identity and in some way contrive to arrest them.”

“Exactly. We must allow them enough leeway, therefore, to be sure they will attempt the crime,” Carter pointed out. “They know what they are up against and that we are out to get them. If we remain too near to Mrs. Thurlow, as if ready to instantly grab any one that lays a finger on her, there will be nothing to it. The miscreants will throw up the job.”

“Surely,” Chick agreed. “No sane man would attempt it under such conditions.”

“The fact that we are carefully disguised, moreover, would not deceive them. They would suspect any men who constantly hung around within reach of Mrs. Thurlow, and would very soon identify us. We must give them enough leeway, therefore, as I have said, to be sure they will make the attempt.”

“I agree with you,” Chick nodded.

“It goes without saying, nevertheless, that we must be in a position to constantly watch the woman,” Carter added. “Having no idea just when the theft may be attempted, we must not lose sight of her for a moment.”

“What plan had we better adopt?”

“We can lay no elaborate plan. It will be of advantage, however, if we keep an eye on one another, as well as on the woman, and contrive to keep her constantly between us. That will enable us to head off a thief in two directions, at least.”

“I see the point.”

“We must be alert, also, to detect any person whose looks or actions warrant suspicion,” Carter continued. “It is barely possible that one of us can discover the crook before the theft is attempted.”

“I’ll put you wise, chief, in that case, and you do the same.”

“Yes, of course.”

“Her nephew is to be her escort, you say.”

“Yes. His name is Dorson. He will accompany both Mrs. Thurlow and her daughter, and we can identify them when they arrive.”

“And our work must begin at that moment.”

“Exactly. Naturally, of course, Dorson will pay considerable attention to Mrs. Thurlow, and I don’t think his presence will deter the crooks, for I have directed her to say nothing to him about expecting a crime. There is no occasion for any one to suspect him, of course, even though he is with her much of the time.”

The detective added the last while they were about to leave. It was a perfectly natural supposition, of course, that the man of whom he was speaking was entirely trustworthy. He did not have a thought to the contrary, and, therefore, he could not foresee the fatal result of this misplaced confidence in Mr. John Dorson.

It was a brilliant scene upon which the two detectives arrived soon after eight o’clock, which they knew would be sufficiently early. The streets adjoining the park in which the handsome new armory building was situated, in the vast hall and drill room, on the second floor of which the ball was to be held, were crowded with costly, brightly lighted automobiles of nearly every type, leaving as rapidly as possible a throng of fashionably clad men and elaborately gowned women, many lavishly adorned with radiant gems and jewels.

Fortune favored the detectives at first. They had been waiting only a few minutes in the broad reception hall on the ground floor, when Carter saw Mrs. Thurlow and Edna arrive in company with a tall, somewhat cadaverous man, who he knew must be Mr. John Dorson.

“There they are, Chick,” he said quietly. “The woman has not weakened. She is doing her part, indeed, to help us nail our man. She is wearing the rope of pearls.”

“Some pearls, too,” Chick muttered admiringly. “By Jove! they warrant taking a desperate chance. That tall fellow is Dorson, I suppose.”

“Surely.”

“He’s not very attractive. He has the look of a rounder.”

“Not as bad as that, I guess,” said Carter. “I think Mrs. Thurlow would have told me. Step down that way and keep an eye on her. We now must watch her constantly.”

Both had been standing in an alcove formed by the rise of the broad, main stairway. The latter led up to a wide corridor flanking three sides of the ballroom, which was accessible from each through several broad, pillared doorways. In the end wall of the room was a row of open French windows, leading out upon the balcony roof of a wide veranda overlooking an avenue through the park mentioned, in which numerous automobiles already had gathered to await the end of the festivities.

One among them had arrived quite early and obtained a position of special advantage, close to the broad avenue and within easy view of the veranda and balcony. It attracted no more attention than any of the others, neither did the chauffeur, who sat motionless at his wheel. None would have recognized his bearded face, nor could the car have been traced from the license number it then appeared to bear.

It was to these conditions and surroundings that Professor Karl Graff had referred while talking with Dorson in the road house, and of which he and his knavish confederates were prepared to take every advantage.

Chick slipped away from his chief, as the latter had directed, and took a position from which he could watch the door of a room to which Mrs. Thurlow and Edna had gone to leave their outside garments, while Dorson hastened to another to check his crush hat and Inverness. Though his face was unusually pale and grave, it wore no expression inviting suspicion.

He returned in a few moments and rejoined Edna Thurlow, departing with her through the throng in the lower corridor and mingling with the stream of wealth and fashion then seeking the ballroom.

Mrs. Thurlow came out a little later and joined a group of women acting as a reception committee, and for nearly an hour she remained in the lower hall, apparently undisturbed by the threats of which she had been informed, and conducting herself precisely as if ignorant of them, as Carter had directed.

Both detectives, though they then were separated, had an eye on her all the while and on the rope of lustrous pearls adorning her shapely neck and perfect shoulders. Neither could detect any person near her inviting suspicion, however, and it really seemed improbable that so daring a theft could be successfully committed, in view of the fact that it had been predicted and prevention audaciously invited.

It was ten o’clock when Mrs. Thurlow went up to the lavishly decorated ballroom. There, and in the adjoining corridors, a throng of several hundred guests were assembled. A dance then was in progress, however, and the corridors were less crowded than during the intervals between the dances.

Carter and Chick met on the stairs while following the woman quite closely, and Carter said a bit hurriedly, noting the direction she was taking:

“She’s going to that end of the hall overlooking the balcony. I’ll follow her. You hurry around through the corridor, so as to watch her from the opposite side of the hall. We then will have her guarded from both directions.”

“Suppose she goes out on the balcony?”

“Slip out through one of the other windows. You must not lose sight of her.”

“I’ve got you,” Chick muttered, as he turned at the head of the stairs and hurried away.

Carter followed the woman in the opposite direction, admiring her outward composure and the nerve she was displaying. He saw her enter the last of the broad doors and thread her way by the throng of dancers, finally halting near one of the windows leading out to the balcony, where she was immediately joined by a colonel of the Guards, in full-dress uniform, and a lady, with whom he had been dancing.

Carter paused in the broad doorway, with a quick and searching glance in each direction. He caught sight of Chick, just entering a door directly across the broad, brightly lighted hall. He saw Edna Thurlow amid the throng of dancers, and noticed that she was pale and paying little attention to the remarks of her partner. He saw, too, the tall form of Mr. John Dorson, who then was standing alone near the second window beyond that near which Mrs. Thurlow had halted.

Though none could know it save the miscreant who had planned the daring job, the situation then was one for which he had been waiting, the crucial moment when conditions assured him of success, when the avenue fronting the veranda was unobstructed, when flight would be easy, when the throng in the ballroom were absorbed in the dance, when the strains of orchestral music drowned all other sounds, and when the victim of his designs had paused at a time and place that perfectly served his purpose.

Two inconspicuous, bearded men in evening dress, who had apparently been talking carelessly on the balcony, suddenly separated.

One of them glided quickly toward the window near which Mrs. Thurlow was standing, taking a position close against the wall.

The other moved in the opposite direction, stopping short near the second window and taking a small electric flash light from his pocket. Hooding it with both hands, so that its glare might not be observed by any of the persons then on the balcony, he lighted the lens for a moment, so holding it that it could be seen from the grounds, on which motionless motor cars then were parked.

The signal was answered almost instantly. The lamps of one of the motionless motor cars shot a quick glare outward over the avenue, and in another moment it was moving moderately in that direction.

The man with a searchlight turned quickly and entered the French window. He passed directly back of Dorson, and, without stopping, whispered hurriedly:

“Now, Dorson, be quick! Get in your work!”

Dorson started as if stung. He did not recognize the bearded man, but there was no mistaking his voice, that fierce, sibilant hiss that he had heard at the road house—the threatening voice of Professor Karl Graff.

Dorson instantly pulled himself together, nevertheless, and nerved himself for what he had undertaken. He took the celluloid box from his pocket, concealing it in his hand, and removed the cover, at the same time walking toward Mrs. Thurlow, at whom he had been gazing when he heard Graff’s threatening command.

When nearly back of her, Dorson stooped to the floor and pretended to pick up a handkerchief—which he had deftly removed from the box, quickly replacing the latter in his pocket.

“Pardon me,” said he, stepping in front of her. “You have dropped your handkerchief, Aunt Clara.”

The colonel talking with her turned at once to his partner, and they whirled away amid other dancing couples.

“My handkerchief, Jack?” Mrs. Thurlow took it, but with a look of surprise.

“I think so.” Dorson drew back a step and with one hand covered his mouth and nostrils.

“No, this is not mine. You are mistaken.”

“Are you sure, Aunt Clara? It was on the floor behind you. I thought you had dropped it.”

Mrs. Thurlow bowed her head a little closer to examine it, still much crumpled, unfolding it and seeking an initial.

“No, it is not mine, Jack,” she repeated. “It may be marked, however, or—or——”

Her voice suddenly died away to a whisper. She looked up at Dorson, as if strangely dazed, and he saw her eyes quickly taking on the vacant expression that had been predicted, the pupils contracting to mere pinpoints, abnormally bright, while her lips turned from red to a dull gray.

Though his every nerve was quivering with secret terror, Dorson kept his head and continued to play his part. He instantly took the woman’s arm, saying quietly:

“You are pale and look tired. Step out on the balcony with me. The air will revive you.”

Mrs. Thurlow obeyed him as if in a trance or a victim of an hypnotic spell. She walked out with him through the French window. There was a large wicker chair near by, and Dorson placed her in it, then whisked the fateful handkerchief from her fingers and thrust it into his pocket. Then he hurried back into the ballroom, through which he passed as if in haste to obtain water, as he really was.

The man lurking near the wall in the dim light instantly approached the woman. Pausing beside her chair, he bowed as if to converse with her. His keen, black eyes shot one swift glance at a few persons on a remote part of the balcony. None was observing him. His deft hands quickly lifted the rope of pearls and dropped it into his pocket. Then he took out a small glass vial, poured the contents of it upon a sponge, and held the latter to the woman’s nostrils for a few seconds.

Mrs. Thurlow gasped and caught her breath.

The man accidentally dropped the vial and it rolled out of sight. He did not wait to search for it, did not dare to delay his departure. He walked quickly toward a corner of the balcony, where the top of a vine-covered trellis rose just above the railing.

Toby Monk was at that moment passing the corner with his motor car.

Both Nick Carter and Chick had witnessed the episode in the ballroom, and the same thought arose in the minds of both—that Mrs. Thurlow was perfectly safe while with her nephew.

The moment that Dorson returned alone, however, both detectives felt a quick thrill of suspicion, an instinctive feeling that the fateful moment had arrived, and both hurried toward the nearest of the French windows, making their way as quickly as possible through the maze of whirling dancers.

Chick was the first to reach the balcony. Coming from the glare in the ballroom, he could not immediately see the seated woman in the dim light outside. He discovered her in a moment, however, and ran toward her—just as his chief hurriedly approached from the opposite direction.

One glance at Mrs. Thurlow’s white face, at her vacant eyes and lax figure, at the neck, then bare of its lustrous adornment—one glance was enough.

“By thunder, they’ve turned the trick!” Chick cried, staring. “That man Dorson must——”

Carter did not wait to hear him. He had swung around like a flash, seeking the thief, knowing that scarce a minute had passed since the woman left the ballroom. The few persons then on the balcony had not observed any disturbance, but the detective instantly caught sight of the swaying top of the trellis mentioned.

He ran in that direction, reaching for his revolver, but he arrived at the corner of the balcony rail only in time to see a slender, black-clad figure leap into a moving motor car, that instantly sped away down the avenue—Tim Hurst, with the rope of pearls in his pocket.