I stood in the foyer of the Astoria Hotel. About me was the pulsing stir of transatlantic life, for the tourist season was now at its height, and I counted myself fortunate in that I had been able to secure a room at this establishment, always so popular with American visitors. Chatting groups surrounded me and I became acquainted with numberless projects for visiting the Tower of London, the National Gallery, the British Museum, Windsor Castle, Kew Gardens, and the other sights dear to the heart of our visiting cousins. Loaded lifts ascended and descended. Bradshaws were in great evidence everywhere; all was hustle and glad animation.  
The tall military-looking man who stood beside me glanced about him with a rather grim smile.
 
"You ought to be safe enough here, Mr. Cavanagh!" he said.
 
"I ought to be safe enough in my own chambers," I replied wearily. "How many of these pleasure-seeking folk would believe that a man can be as greatly in peril of his life in Fleet Street as in the most uncivilized spot upon the world map? Do you think if I told that prosperous New Yorker who is buying a cigar yonder, for instance, that I had been driven from my chambers by a band of Eastern assassins founded some time in the eleventh century, he would believe it?"
 
"I am certain he wouldn't!" replied Bristol. "I should not have credited it myself before I was put in charge of this damnable case."
 
My position at that hour was in truth an incredible one. The sacred slipper of Mohammed lay once more in the glass case at the Antiquarian Museum from which Earl Dexter had stolen it. Now, with apish yellow faces haunting my dreams, with ghostly menaces dogging me day and night, I was outcast from my own rooms and compelled, in self-defence, to live amid the bustle of the Astoria. So wholly nonplussed were the police authorities that they could afford me no protection. They knew that a group of scientific murderers lay hidden in or near to London; they knew that Earl Dexter, the foremost crook of his day, was also in the metropolis—and they could make no move, were helpless; indeed, as Bristol had confessed, were hopeless!
 
Bristol, on the previous day, had unearthed the Greek cigar merchant, Acepulos, who had replaced the slipper in its case (for a monetary consideration). He had performed a similar service when the bloodstained thing had first been put upon exhibition at the Museum, and for a considerable period had disappeared. We had feared that his religious pretensions had not saved him from the avenging scimitar of Hassan; but quite recently he had returned again to his Soho shop, and in time thus to earn a second cheque.
 
As Bristol and I stood glancing about the foyer of the hotel, a plain-clothes officer whom I knew by sight came in and approached my companion. I could not divine the fact, of course, but I was about to hear news of the money-loving and greatly daring Graeco-Moslem.
 
The detective whispered something to Bristol, and the latter started, and paled. He turned to me.
 
"They haven't overlooked him this time, Mr. Cavanagh," he said. "Acepulos has been found dead in his room, nearly decapitated!"
 
I shuddered involuntarily. Even there, amid the chatter and laughter of those light-hearted tourists, the shadow of Hassan of Aleppo was falling upon me.
 
Bristol started immediately for Soho and I parted from him in the Strand, he proceeding west and I eastward, for I had occasion that morning to call at my bank. It was the time of the year when London is full of foreigners, and as I proceeded in the direction of Fleet Street I encountered more than one Oriental. To my excited imagination they all seemed to glance at me furtively, with menacing eyes, but in any event I knew that I had little to fear whilst I contrived to keep to the crowded thoroughfares. Solitude I dreaded and with good reason.
 
Then at the door of the bank I found fresh matter for reflection. The assistant manager, Mr. Colby, was escorting a lady to the door. As I stood aside, he walked with her to a handsome car which waited, and handed her in with marks of great deference. She was heavily veiled and I had no more than a glimpse of her, but she appeared to be of middle age and had gray hair and a very stately manner.
 
I told myself that I was unduly suspicious, suspicious of everyone and of everything; yet as I entered the bank I found myself wondering where I had seen that dignified, grayhaired figure before. I even thought of asking the manager the name of his distinguished customer, but did not do so, for in the circumstances such an inquiry must have appeared impertinent.
 
My business transacted, I came out again by the side entrance which opens on the little courtyard, for this branch of the London County and Provincial Bank occupies a corner site.
 
A ragged urchin who was apparently waiting for me handed me a note. I looked at him inquiringly.
 
"For me?" I said.
 
"Yes, sir. A dark gentleman pointed you out as you was goin' into the bank."
 
The note was written upon a half sheet of paper and, doubtin............