Philip MacDonald adapted his own novel about a jewel thief who helps Scotland Yard catch the murderer of seven policeman while he romances the commissioner's daughter.
A fifth policeman is murdered with a sword in as many days. Sir Herbert Frensham (Henry Stephenson) is criticized for not being in London and returns with his daughter Jane (Elizabeth Allan), who is met by her fiancé Chris Marche (Ralph Forbes). Superintendent Connor (Lewis Stone) shows Frensham five postcards from X, and they get a sixth. That night a policeman is stabbed and seen by Nick Revel (Robert Montgomery) after he steals a diamond. Connor suspects the diamond thief. Taxi driver Palmer (Forrester Harvey) tells Nick to throw it in the Thames; but 10,000 pounds is offered for it. A drunk Chris takes a policeman's hat, knocking him down and leaving his scarf. Then the policeman is killed. Jane identifies the scarf, and Chris is arrested. Nick has an idea to catch the murderer; he testifies at the hearing and is confirmed by Palmer. The murder charge is dismissed, and the judge sentences Chris to ten days for assault. Nick goes to lunch with Jane, who suspects he lied. Nick bets her that her father will consider his idea; but Connor and Frensham worry about deceiving the press. Connor investigates Nick and suspects he has the diamond.
Nick attends a dinner for Chris. Frensham collapses at a press conference. Chris becomes jealous of Nick. Another policeman is stabbed as Connor has Nick's flat searched. Nick takes the diamond from a lamp, and Connor walks in. They go out, and Nick is accused of taking a wallet; but he passes the diamond to Palmer with a pound note. Willis tells Nick that Palmer gave him the diamond, and he puts it in Nick's glass. Nick is being followed, but Jane gives him a ride home. Chris comes there, finds Jane, and ends their engagement. Jane likes Nick for being straight; but he says he is a fraud. Newspapers say Frensham must go. Nick tells Palmer he is sending the diamond to Frensham at Scotland Yard from X and is going straight. Connor questions Hutchinson (Ivan F. Simpson) about the diamond that was insured for 50,000. Nick figures out where X will strike next by plotting his murders on a map. He gets a police costume and leaves his apartment even though it is surrounded by police looking for him. Connor finds the map and follows Nick. Nick gives Palmer a pistol, and he breaks X's sword in a gate. X shoots at Nick, who drops his gun. Nick fights with X, as the police are arriving. X falls down an elevator shaft. Connor arrests Nick; but X confesses he wanted to kill a policeman for each of his fifteen years in prison; then he dies. In the final scene Connor presents Nick and the diamond to the press, and Nick leaves with Jane.
Sophisticated comedy lightens this murder mystery about an insane criminal's revenge against law officers. Love turns Nick from a thief to a hero.