powered by AFI
After four constables are stabbed to death in four different sections of London by a serial killer who calls himself "Mr. X," Sir Herbert Frensham, the beleagured police commissioner, confers with Inspector Connor about how to stop the killings. With no true leads, however, the police fail to prevent a fifth murder, which occurs at the same time and near the same place as the robbery of the valuable Drayton diamond. Aware that the police believe that Mr. X has the diamond, the true thieves, Nick Revel and his cohorts--taxi driver Joseph Horatio Palmer and insurance clerk Hutchinson--decide to wait until Mr. X is apprehended before collecting a 10,000 pound insurance reward on the gem. Soon after, a sixth murder occurs, and Sir Christopher Marche, the fiancé of the commissioner's daughter Jane, is arrested because he had drunkenly assaulted the constable just before he was slain. Anxious to speed up Mr. X's capture, Nick and Palmer conspire to testify on Marche's behalf and state in court that, while driving by in Palmer's cab, they saw Marche leave the scene before the murder was committed. Nick then ingratiates himself with Jane and connives to have dinner with her father in order to suggest a way of catching the killer. Although Sir Herbert is impressed by Nick's idea of trapping Mr. X by announcing in the newspapers that his last victim survived the attack and can identify his attacker, Connor is suspicious of the thief and orders a background check on him. While Nick romances Jane away from Marche, Connor searches Nick's flat and tries to catch him with the jewel by having an undercover policeman accuse him of theft. Before he is taken to police headquarters, however, the wiley Nick passes the diamond to Palmer, who in turn gives the jewel to Hutchinson. In a pub, a nervous Hutchinson returns the jewel by dropping it in Nick's beer, which Nick then swallows under the nose of an undercover policeman. Although Nick eludes the police by driving off with an unsuspecting Jane, Connor sends the already jealous Marche to Nick's flat. Outraged by Jane's presence in Nick's flat, Marche breaks their engagement, and Nick, realizing that he loves Jane, decides to end his life of crime and return the diamond. After Nick mails the jewel with a typed note from "Mr. X," Connor picks up Hutchinson and begins to interrogate him. As Hutchinson nears the point of confession, Nick deduces that Mr. X, who has announced his next murder for that night, has been using an "x" pattern to determine his murder locations. Confident he knows where Mr. X will strike next, Nick disguises himself as a constable, sneaks out of his police-guarded flat and rushes across town. After a fierce fight, Nick fatally wounds Mr. X, who then confesses to the police before dying that he committed the murders as revenge for a previous conviction. Hailed as a hero, the reformed Nick poses for reporters' photographs with the now-vindicated commissioner and embraces a loving Jane.