CHAPTER I

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      "A GENTLEMAN to see you, Doctor."

      From across the common a clock sounded the half-hour.

      "Ten-thirty!" I said. "A late visitor. Show him up, if you please."

      I pushed my writing aside and tilted the lamp-shade, as footsteps sounded on the landing. The next moment I had jumped to my feet, for a tall, lean man, with his square-cut, clean-shaven face sun-baked to the hue of coffee, entered and extended both hands, with a cry:

      "Good old Petrie! Didn't expect me, I'll swear!"

      It was Nayland Smith — whom I had thought to be in Burma!

      "Smith," I said, and gripped his hands hard, "this is a delightful surprise! Whatever — however — "

      "Excuse me, Petrie!" he broke in. "Don't put it down to the sun!" And he put out the lamp, plunging the room into darkness.

      I was too surprised to speak.

      "No doubt you will think me mad," he continued, and, dimly, I could see him at the window, peering out into the road, "but before you are many hours older you will know that I have good reason to be cautious. Ah, nothing suspicious! Perhaps I am first this time." And, stepping back to the writing-table he relighted the lamp.

      "Mysterious enough for you?" he laughed, and glanced at my unfinished MS. "A story, eh? From which I gather that the district is beastly healthy — what, Petrie? Well, I can put some material in your way that, if sheer uncanny mystery is a marketable commodity, ought to make you independent of influenza and broken legs and shattered nerves and all the rest."

      I surveyed him doubtfully, but there was nothing in his appearance to justify me in supposing him to suffer from delusions. His eyes were too bright, certainly, and a hardness now had crept over his face. I got out the whisky and siphon, saying:

      "You have taken your leave early?"

      "I am not on leave," he replied, and slowly filled his pipe. "I am on duty."

      "On duty!" I exclaimed. "What, are you moved to London or something?"

      "I have got a roving commission, Petrie, and it doesn't rest with me where I am to-day nor where I shall be to-morrow."

      There was something ominous in the words, and, putting down