It was no mere chance that took Scott Egerton, the brilliant young politician and amateur sleuth, to the Poisson D'Or, that rather dubious inn in an obscure quarter of Paris, on the night that its proprietor expected Sir Hector Ascher, owner of the unique and priceless Ascher Library, and a man of considerable substance. On Ascher's arrival at a late hour, traveling incognito and bringing with him a number of extremely valuable manuscripts, it was obvious that he was in a state of appalling terror, having, he declared, been followed from London to Paris. He insisted on having his window barred and his door double-locked, but despite these precautions, on the following morning, the window was discovered to have been opened from the inside, although the door was still fastened, while Ascher's body was found stabbed near the unused bed and the manuscripts had disappeared.

Suspicion shifted from one quarter to another until it seemed probable that the real criminal would escape detection, but Scott Egerton, by a process of logical deduction, succeeded in foisting the blame for an atrocious crime where it actually belonged and in solving the baffling problem of who did open the window.