All this was merely deduction, for so far as I had seen, “J.M.’s” travelling companions hadn’t even accosted him.  Still, the theory accounted for much that had been puzzling, and made it plausible that a man should be desperate enough to trust his treasure to a stranger (known only through “photos in the newspapers”) rather than risk losing it to those he had betrayed.

I resolved to use all my powers of diplomacy to extract from “J.M.” the case containing the treaty before he learned that he was not to receive the diamonds in its place; and I had no more than vaguely mapped out a plan of proceeding before I arrived in the Avenue Morot.  Thence I soon found my way into the Rue de la Fille Sauvage, a mean street, to which the queer name seemed not inappropriate.  The house I had to visit was an ugly big box of a building, with rooms advertised to let, as I could see by the light of a street lamp across the way, which gleamed bleakly on the lines of shut windows behind narrow iron balconies.

The large double doors, from which the paint had peeled in patches, were closed, but I rang the bell for the concierge; and after a delay of several minutes I heard a slight click which meant that the doors had opened for me.  I passed into a dim lobby, to be challenged by a sleepy voice behind a half open window.  The owner of the voice kept himself invisible and was no doubt in the bunk which he called his bed.  Only a stern sense of duty as concierge woke him up enough to demand, mechanically, who it was that the strange monsieur desired to visit at this late hour?

I replied according to instructions.  I wished to see Monsieur Gestre.

“Monsieur Gestre is away,” murmured the voice behind the little window.

I thought quickly.  Gestre was probably the “pal” whom “J.M.” had been in such a hurry to find.  “Very well,” said I, “I’ll see his friend, the Englishman who arrived this evening.  I have an appointment with him.”

“Ah, I understand.  I remember.  Is it not that Monsieur has been here already?  He now returns, as he mentioned that he might do?”

Again my thoughts made haste to arrange themselves.  The “monsieur” who had called had probably also arrived late, after the concierge had gone to bed in his dim box, and become too drowsy to notice such details as the difference between voices, especially if they were those of foreigners.  Perhaps if I explained that I was not the person who had said he would come again, but another, the man behind the window would consider me a complication, and refuse to let me pass at such an hour without a fuss.  And of all things, a fuss was what I least wanted—­for Maxine’s sake, and because of the treaty.  I decided to seize upon the advantage that was offered me.

“Quite right,” I said shortly.  “I know the way.”  And so began to mount the stairs.  Flight after flight I went up, meeting no one; and on the fifth floor I found that I had reached the top of the house.  There were no more stairs to go up.