“He had latterly developed symptoms of angina pectoris,” explained the family physician; “but I had not anticipated a fatal termination so soon.  I was called about two o’clock this morning, and found Lord Southery in a dangerously exhausted condition.  I did all that was possible, and Sir Frank Narcombe was sent for.  But shortly before his arrival the patient expired.”

“I understand, Doctor, that you had been treating Lord Southery for angina pectoris?” I said.

“Yes,” was the reply, “for some months.”

“You regard the circumstances of his end as entirely consistent with a death from that cause?”

“Certainly.  Do you observe anything unusual yourself?  Sir Frank Narcombe quite agrees with me.  There is surely no room for doubt?”

“No,” said Smith, tugging reflectively at the lobe of his left ear.  “We do not question the accuracy of your diagnosis in any way, sir.”

The physician seemed puzzled.

“But am I not right in supposing that you are connected with the police?” asked the physician.

“Neither Dr. Petrie nor myself are in any way connected with the police,” answered Smith.  “But, nevertheless, I look to you to regard our recent questions as confidential.”

As we were leaving the house, hushed awesomely in deference to the unseen visitor who had touched Lord Southery with gray, cold fingers, Smith paused, detaining a black-coated man who passed us on the stairs.

“You were Lord Southery’s valet?”

The man bowed.

“Were you in the room at the moment of his fatal seizure?”

“I was, sir.”

“Did you see or hear anything unusual—­anything unaccountable?”

“Nothing, sir.”

“No strange sounds outside the house, for instance?”

The man shook his head, and Smith, taking my arm, passed out into the street.

“Perhaps this business is making me imaginative,” he said; “but there seems to be something tainting the air in yonder—­ something peculiar to houses whose doors bear the invisible death-mark of Fu-Manchu.”

“You are right, Smith!” I cried.  “I hesitated to mention the matter, but I, too, have developed some other sense which warns me of the Doctor’s presence.  Although there is not a scrap of confirmatory evidence, I am as sure that he has brought about Lord Southery’s death as if I had seen him strike the blow.”

It was in that torturing frame of mind—­chained, helpless, in our ignorance, or by reason of the Chinaman’s supernormal genius—­that we lived throughout the ensuing days.  My friend began to look like a man consumed by a burning fever.  Yet, we could not act.

In the growing dark of an evening shortly following I stood idly turning over some of the works exposed for sale outside a second-hand bookseller’s in New Oxford Street.  One dealing with the secret societies of China struck me as being likely to prove instructive, and I was about to call the shopman when I was startled to feel a hand clutch my arm.