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. When a well known and respected man named Sir Christopher Marche is arrested for the murders on circumstantial evidence, Nick knows the police have the wrong man. Nick, who has an idea on how the police can find Mr. X, decides the best thing he can do is to make a up a story to clear Sir Christopher while suggesting his plan to the police. In this process, both Sir Christopher and the lead investigator, Superintendent Connor, don't quite trust Nick, the latter who suspects he is the diamond thief. Nick realizes that the police are onto him, which worries both Hutch and Joe more than it worries the confident Nick. But the one person who does trust Nick is Jane Frensham, Sir Christopher's fiancé and Police Commissioner Sir Herbert Frensham's daughter. Sir Herbert himself has been under great criticism by the public for failing to stop the murders. In the process of Nick's assistance, Nick and Jane have fallen in love with each other. To protect himself under the circumstances and protect Jane's good name, Nick feels his only solution is to capture Mr. X himself.