“That was the question which we had to answer. There is only one possible way. You are aware that the Underground runs clear of tunnels at some points in the West End. I had a vague memory that as I have travelled by it I have occasionally seen windows just above my head. Now, suppose that a train halted under such a window, would there be any difficulty in laying a body upon the roof?”

“It seems most improbable.”

“We must fall back upon the old axiom that when all other contingencies fail, whatever remains, however improbable, must be the truth. Here all other contingencies have failed. When I found that the leading international agent, who had just left London, lived in a row of houses which abutted upon the Underground, I was so pleased that you were a little astonished at my sudden frivolity.”

“Oh, that was it, was it?”