There have been a spate of London police murders, the victims always killed by a long knife (which the police know is a sword cane), the murders always taking place in a deserted but successively different part of town, and the police always being notified by the murderer that he will strike beforehand. By his signed notes, the murderer has dubbed himself as X. The police have their first real lead when the latest murder occurs outside the home where the Drayton Diamond was stolen on the same night. The diamond thief is Nick Revel, a suave, confident career thief, and his two accomplices, an insurance clerk named 'Hutch' Hutchinson, and a taxi driver named Joe Palmer. Nick, however, is not Mr. X, but he and his accomplices know they can't pawn the diamond or return the diamond for its insurance money now until Mr. X is caught.