There was something ominous in the continuous peal that sounded louder and louder in my ears with every step I made towards it. It seemed as though the unknown caller must know of my predicament and be bent on exposing me. I clutched the rail of the banisters to steady myself as I panted up those interminable stairs in the darkness, and my feet felt clogged like those of one in a nightmare as I lifted them from step to step; all the while racking my brains for some excuse to offer for the breach of duty I had been guilty of in spending the night elsewhere. For my real excuse, the only one that could have tempted me to betray my chief’s confidence, could never be disclosed. The darkness all around me seemed to be vibrating with the merciless clamour overhead as I toiled through those tense moments. My knees trembled under me, and my heart well-nigh stopped beating, as my head reached the level of the last landing and I turned my eyes desperately to the physician’s door in search of any sign that he had been aroused. No sign as yet, thank Heaven! Five more stairs, three lightning strides to my own door, and he would never know of the secret errand that had taken me away from my post that night. At last my agony was ended. I stood breathless on the topmost stair, darted past Sir Frank’s room, not daring to pause and listen for any movement from within, and clutched the handle of my own door, summoning all my nerve to open and close it again so rapidly as to permit the least possible sound to escape. An instant later and I had reached the telephone and silenced its urgent voice, and was beginning to draw my breath freely for the first time since I had reached the house. Then, after a few deep gasps, I hailed the caller. “Inspector Charles of Scotland Yard speaking,” came grimly over the wire. “Who is there?” There was nothing to startle me in the fact that the police were calling so imperatively. Tarleton was the greatest living authority on poisons; it was to pursue his researches in their mysterious history that he lived in the unfashionable neighbourhood of the Museum; and the Home Office treated him with a confidence which they placed in no other of their advisers. Neither was there any cause for uneasiness in the Inspector’s cautious question. Very many of the calls that came to that unpretending house in that quiet corner of London had a certain character of furtiveness, and the callers showed the same anxiety to make sure whom they were speaking with. My usual response came to my lips mechanically. “This is Dr. Cassilis, Sir Frank Tarleton’s confidential assistant. The doctor is asleep.” There was a pause before the caller spoke again. There was nothing alarming to me in that, either. I had grown accustomed to the pause during my first few weeks under the roof of the great consultant. Few of those who needed his services liked to disclose their business to a deputy.