Michael Lanyard, a reformed jewel thief known as The Lone Wolf, nearly kills a woman while speeding to a winter resort. Stopped by the police, Lanyard and his personal valet, Jamison, along with the woman, Joan Bradley, are hauled off to police headquarters for questioning. En route, however, they manage to give the officer the slip, and Joan confides to Lanyard the part she has just played in a murder. Joan tells Lanyard that her ex-husband, Pete Rennick, whom she believed was dead, visited her on the eve of her marriage to millionaire Bob Penyon. While attempting to steal the diamond necklace that Bob gave her as a wedding gift, Pete was killed by a mysterious gunshot. Lanyard is intrigued by Joan's story and helps her buy time by furnishing her with an alibi. When Bob and the police investigators arrive, the police discover an inconsistency in her story and believe that she is hiding something. Secretly, Bob and Joan, who believe that they are about to be arrested, flee the apartment. Later, at Lanyard's home, Lanyard and Jamison are visited by Clay Beaudine, the mastermind behind the attempted robbery. Beaudine, convinced that Lanyard has reverted to his criminal past and is involved in the theft, insists that he enter into a partnership with him, but Lanyard resists his "offer." He then seeks the professional advice of his friend, Nick, a jeweler. Shortly after, Lanyard, posing as Inspector Crane, questions members of the Penyon family in their home. One member of the family, Rose Waverly, appears highly suspect in Lanyard's opinion, and he eventually coaxes her into confessing that she tipped off Beaudine in exchange for money that she desperately needed. Meanwhile, back at Lanyard's apartment, Beaudine forces his way in, and when he sees a newspaper article about the murder and recognizes the man in the picture, he locks Jamison and Joan in the bathroom. He then calls the man in the picture and arranges a rendezvous with him. When Lanyard returns home, he traces the address of Beaudine's destination through a dry cleaning ticket and follows him there, only to find Beaudine's recently murdered body. When Lanyard learns from Nick that the necklace was broken up and sold in pieces a year ago, he sets a trap for the killer by having Joan call Peter Van Wyck, a friend of the Penyons', to tell him that the jewels have been discovered at the bottom of a river and are about to be dredged up. Soon after Van Wyck arrives, Lanyard accuses him of the stealing the fake jewels to commit insurance fraud and of killing Pete to cover up the theft. Frightened, Van Wyck takes Joan hostage at gunpoint, but he is shot by Crane's quick-thinking assistant. The mystery solved, Joan and Bob are advised by Mrs. Penyon to marry as soon as possible, before some new intrigue spoils their wedding plans.