London, an evening of heavy snowfall. A certain Maureen Lyon has been strangled at 24 Culver Street, not far from Monkswell Manor, a guest house run by the Ralstons who are welcoming their first guests.

The following afternoon, on arriving at the scene, Inspector Trotter reveals that the murderer, in his flight, dropped a piece of paper with the boarding house's address on it. He must therefore be among them. The investigation begins...