Nick Carter finished his breakfast at eight o’clock the following morning. He needed no one to tell him that Patsy Garvan, who still was absent, had fallen into the hands of the remarkably clever and thus far successful gang he was seeking. It was only half an hour later when Carter entered the Osgood Hospital, where he was received in the business office by Jim Shannon, then in his customary livery.

“Doctor Devoll is not here, sir,” he said respectfully, in reply to the detective’s question. “He seldom comes here before noon. He has outside patients, sir, and other business. You might catch him before he goes out, sir, if your business is important.”

“Out from where?” Carter asked curtly.

“From his apartments, sir. He has a suite in the Pemberton.”

“Where is that?”

“About ten minutes’ walk from here,” Shannon said suavely. “I can find out for you, sir, whether he is there.”

“By telephone?”

“Yes, sir.”

“Do so,” the detective said shortly.

He sat down and kept an eye on the man, who did not appear in the least disturbed by the detective’s visit. One less quick to suspect subterfuge would have apprehended that his suspicions were misplaced, that Shannon knew nothing about the anonymous letter, and that Doctor Devoll was not the sender of it, after all.

Nick Carter, however, had no such apprehension. He knew that he was up against as cool and crafty a gang of knaves as ever stood in leather. He now was accepting nothing that appeared on the surface. He was seeking the wheel within.

He watched and listened while Shannon telephoned, readily getting Doctor Devoll on the wire and stating that Mr. Blaisdell, who had called the previous day, would like to come to the Pemberton to see him. That was all that Shannon said, noncommittal it was, too, and he immediately hung up the receiver and turned to the detective.

“Yes, sir, Doctor Devoll is there, and it’s all right,” he said, with the air of one glad to have conferred a favor. “He will wait for you. You can go right up.”

Nick took all this for what he thought it was worth. He lingered only to inquire the way, then turned on his heel and departed.

Shannon watched him hasten across Hamilton Square, and then, with a scowl as black as a thunder-cloud, he darted to the telephone.

Ten minutes had passed when the detective knocked on the door of a second-floor suite in the Pemberton, and he was immediately admitted by the man he was seeking.

Doctor Devoll looked more lean and bald than usual in the sunlight shed into his attractively furnished parlor. He wore a short, velvet jacket, his customary black vest and trousers, and he greeted the detective with an ingratiating smile.

“Come in, Mr. Blaisdell, and take a seat,” he said, waving Carter to a chair. “I remembered your visit, of course, when Shannon called me up. You were very lucky, however, in finding me this morning.”

“Yes?” queried Carter tentatively.

“I usually leave here about half past eight, but I overslept this morning. I was very busy at the hospital all of last evening, and did not retire till after midnight.”

“A serious case or an operation?”

“Neither. I was doing some writing in my private room, with the help of my attendant,” Doctor Devoll explained blandly. Then he added, with a covert leer deep down in his squinted eyes: “But it’s an ill wind, indeed, that blows no one any good. What can I do for you, Mr. Blaisdell?”

Nick Carter heard him without a change of countenance, but with no faith in the alibi so quickly volunteered. He remembered the location of the physician’s room, the strict privacy that was possible, and his grounds for having suspected Shannon of duplicity. He felt sure that they already had framed up a story to show, if it became necessary, that they were not on the scene of the robbery the previous evening.

“You can, I think, give me some very desirable information,” Carter replied, with steadfast scrutiny. “Speaking of doing some writing, Doctor Devoll, have a look at this anonymous letter. Read it, please, and tell me what you think of it.”

Doctor Devoll took it, smiling, and glanced at the address.

“Dear me!” he exclaimed, looking up quickly. “It is addressed to Nick Carter.”

“I am Nick Carter.”

“The famous detective?”

“I am a detective.”

“Well, well, this is most surprising.” Devoll appeared greatly astonished. “I thought your name was Blaisdell. Why are you using a fictitious name? What could——”

“I will presently explain,” Nick interrupted. “Kindly read the letter.”

Doctor Devoll complied. Nothing denoted that he was reading his own threatening letter. His crafty face took on, instead, a look of mingled wonderment and indignation.

“Goodness!” said he, gazing straight at Nick. “This is most amazing. A robbery predicted and your life threatened. What audacity! What daring knavery!”

“I agree with you.”

“Do you know who sent it or suspect?”

“I do not. Can you help me?”

“Help you? What a question! Why had you any such idea?” Doctor Devoll demanded, frowning. “I cannot imagine who would send you such a letter.”

“I thought you might know the hand.”

“It is not familiar to me. Why did you think so?”

“I will presently tell you,” said Carter. “The sender has in one respect made good. Mrs. Thurlow’s rope of pearls was stolen last evening.”

“Good heavens, is it possible?” Devoll’s brows rose again with a look of surprise. “In that case, Mr. Carter, you have only one course.”

“What is that?”

“That stated in this anonymous letter. No sane man would ignore such a warning. Leave Madison as quickly as possible. Otherwise, the sender may again make good and kill you. I would advise you to lose no time in returning to New York.”

“I shall do nothing of the kind.”

“No?”

“I shall remain in Madison until I have stuffed that letter down the sender’s throat.”

“Well, that’s up to you, of course, and I admire your nerve.” Doctor Devoll smiled again and returned the letter. “It strikes me, however, that you will take a desperate chance, a foolhardy one, in view of the threat that has been executed. I would expect, if I were in your shoes, to have my head blown off at any moment.”

“I’ll risk it.”

“As I have said, then, it’s up to you.” Doctor Devoll drew forward in his chair and spread his hands on his knees. “But why have you called to show me the letter, and what do you expect to learn from me? I know nothing about it or about the theft of the pearls.”

Nick glanced down at the physician’s hands. He noticed that they were white and slender, that the nails were neatly manicured, and that that on his right thumb was a bit discolored, as if from a slight bruise. He looked up and replied:

“On the contrary, Doctor Devoll, you do know something about the theft.”

“Nonsense! What do you mean by that?”

“Just what I said.”

Doctor Devoll did not reply immediately. He sat meeting the detective’s searching scrutiny without a sign of flinching. His narrowed eyes were taking on a threatening glint, instead, and he said a bit sharply:

“If you repeat that assertion, Mr. Carter, I will order you out of my apartments. I insist that I know nothing about that letter or about the robbery. If you think I am lying——”

“One moment,” Nick interposed, checking him. “Don’t misunderstand me or go over the traces. You will presently agree with me, Doctor Devoll.”

“Agree with you?”

“You have not forgotten, of course, the four girls found unconscious in the hospital grounds.”

“No, certainly not.”

“You treated all of them successfully, but you let them go without making an investigation. Now, Doctor Devoll, I happen to know that their abnormal condition was due to inhaling a powerful narcotic of some kind from a handkerchief found in a small leather purse or bag.”

“Ah! You know more about it, then, than I do.”

“I know, too, that Mrs. Thurlow was overcome by like means and robbed. I also know that the thief administered an antidote that soon revived her—presumably the same antidote that you administered to the four girls. That is why I said that you know something, at least, about the robbery.”

“You mean——”

“I mean that you know, of course, of what the antidote consists,” Nick cut in again. “Otherwise, you would not have used it. That is a logical conclusion, isn’t it?”

“Perfectly—if your premises are correct.”

Doctor Devoll did not appear at all disturbed. If these unexpected discoveries of the detective alarmed him, he did not betray the fact. Only the gleam that shone in his narrow eyes was steadily becoming brighter—and Nick saw and rightly interpreted it.

“They are correct, doctor, all right,” he replied a bit grimly. “If you——”

“Wait!” Doctor Devoll spoke more suavely. “I now see what you meant, Mr. Carter, and at what you are driving. I beg to assure you, too, that I would be very glad to aid you in this matter or give you any information I possess.”

“I had no doubt of that, of course,” Nick said dryly.

“I hope not.” Doctor Devoll smiled again. “But why do you infer that the restorative I used was the same as that given to Mrs. Thurlow. I may have employed only an ordinary stimulant.”

“I doubt that an ordinary stimulant would have been effective,” the detective returned. “Furthermore, a policeman who was present in the case of the last girl saw you saturate a sponge with an amber-colored fluid poured from a small fluted vial. Here is one like it, Doctor Devoll. You may recognize it.”

Doctor Devoll’s nerve did not weaken for an instant. He merely glanced at the vial Nick was displaying, and said blandly:

“You should not have said recognize it, Mr. Carter, for that implies ownership. I never saw that vial before. I admit, however, that I have one precisely like it.”

“And that it contained the antidote you used?”

“Yes.”

“What was it?”

“I don’t know.”

“Don’t know?” Nick echoed incredulously. “Do you mean to assert, Doctor Devoll, that you blindly used——”

“Oh, I admit that it sounds incredible,” Doctor Devoll interrupted. “It is true, sir, nevertheless. The vial and its contents were given to me by a friend, a chemist in whom I have absolute confidence, with directions how and in what cases to use it. I tried it successfully on the first of the four girls, and I since have repeatedly used it. I have not yet learned, however, what ingredients the fluid contained or how it is compounded.”

“Speaking plainly, Doctor Devoll, that story——”

“Oh, I see you are still incredulous,” the physician again interrupted. “It is not surprising, Mr. Carter, under the circumstances. But there is one way to settle it. You can easily verify my statements. Go with me to my friend and he will corroborate——”

“Where must we go?” the detective cut in.

“Not far. He has an office and laboratory in the Waldmere Chambers.”

“H’m, is that so? Who is he?”

“Professor Karl Graff.”

“Humph!” Nick ejaculated. “I remember him.”

He now recalled for the first time, in fact, the elderly man who had approached from the rear of the corridor in which the corpse of the mysteriously murdered Gaston Todd was lying. He remembered the negative statements this man had made. He recalled, too, Patsy Garvan’s description of the gray-bearded man seen at Leary’s road house and the mysterious killing of Leary’s cat. All this flashed upon him with sudden startling significance, giving color to the physician’s story—though Nick decided to keep an eye on him.

“That’s a good idea,” he said abruptly. “Get ready at once. We will go together and see him.”

Doctor Devoll complied with alacrity. A leer lurked in his eyes when he hastened into his bedroom. He quickly returned, wearing his black frock coat and tall silk hat.

“Now, Mr. Carter, I am ready,” he said, smiling. “I will speedily set myself right in your estimation.”

Nick had convictions to the contrary, but he did not express them. In reality, nevertheless, he was considerably puzzled by the increasing complications, and he began to suspect that Professor Karl Graff might be the guilty man, after all—the discoverer of the potent narcotic that had made possible the long series of mysterious crimes.

It was ten o’clock when they entered the Waldmere Chambers and hastened up to the second-floor corridor, toward the rear of which Doctor Devoll conducted the detective, remarking agreeably:

“This way to Professor Graff’s office. We are old friends, and I frequently call here to see him. I have known him for years.”

Carter followed him, with a glance at the spot where Gaston Todd had been found dead, scarcely twenty feet from the door opened by the physician. He led the detective in, and a man arose from a table at which he appeared to have been at work—Tim Hurst.

“Ah, good morning, doctor,” he said respectfully, hastening to place chairs for both visitors.

“Good morning, Tim,” Doctor Devoll said familiarly. “Is Karl in his laboratory?”

“No, sir.” Hurst appeared as frank as a schoolboy. “He has not come down yet. He has not been coming in much before noon lately, sir.”

“Ah, well, I can expedite matters,” Devoll said glibly. “Sit down, Mr. Carter, while I ring him up. His telephone is in the laboratory.”

He passed out of a side door while speaking, and Nick did not detain him, supposing he had merely entered an adjoining room. The door closed automatically. Tim Hurst tendered a morning newspaper, asking politely:

“Have you read the news, sir? There was another robbery last night, Mrs. Mortimer Thurlow, sir, the swell society woman.”

“Yes, I know about it,” Nick nodded, sizing Hurst up more intently. “How long have you been in Professor Graff’s employ?”

“About a year, sir; ever since he came here.”

“He is not an old resident of Madison, then?”

“No, sir. He came here a year ago next month.”

“Where from?”

“I am not sure, sir, but I think he—ah, he is coming right now, sir,” Hurst broke off abruptly. “That’s his step in the corridor.”

Professor Graff entered at that moment, wearing a baggy plaid suit, his overcoat and cape, and with a rusty felt hat on his gray head. His bearded face took on a look of mild surprise when he saw the detective, who immediately arose, while Tim Hurst explained glibly:

“This gentleman came with Doctor Devoll to see you. The doctor has gone down to the laboratory to telephone to you, thinking——”

“We’ll go down, Timothy, and save him the trouble,” Professor Graff interposed blandly, dropping his coat and cape over a chair. “Will you go with us, sir, or——”

“I think I will,” Nick put in, bent upon keeping the physician under his eye, and noting that the chemist did not appear to recall him.

Professor Graff led the way, Nick following, and Tim Hurst bringing up in the rear. Half a minute took them down the stairs, through the basement entry, and into the laboratory.

The detective flashed a swift glance around the room, at the zinc-covered table, the bottle-laden shelves, the ground-glass windows, and at a telephone on one of the walls. But he failed to see the suspected physician, and he drew back a step, instinctively reaching for his revolver.

Graff turned at the same moment, however, and thrust a weapon nearly under the detective’s nose.

“Don’t stir, Carter, foot or finger!” he commanded sternly. “If you do, you’ll be a dead one on the instant. I’ll send a bullet through your meddlesome head.”

Nick Carter was surprised, but not entirely, by the sudden threatening situation. His eyes were turned, not upon Graff’s bearded face, but upon his revolver and the rigid hand that held it—and upon the slightly discolored nail of his right thumb.

Nick recalled where he last had seen it. His gaze leaped up to the bearded face. In spite of beard and wig and slouch hat and padded coat, he now discovered the wheel within. He was gazing not at the remarkably artistic disguise, but, through it, at the thin face and threatening eyes of—Doctor David Devoll.