At the Penyon mansion, Warren Hull as Bob Penyon presents his fiancée, Jean Muir as Joan Bradley, with a priceless diamond necklace, a family heirloom. Marla Shelton as Rose Waverly phones Beaudine to say the couple has just left with the necklace. Peter Rennick, the husband Joan believed dead, would rather blackmail his wife, but Victor Jory as Clay Beaudine wants the necklace for himself. Rennick confronts Joan and forcibly takes the necklace, but is shot and killed by an unseen murderer, who takes the jewelry. Panicking, Joan flees her apartment. Waiting outside, Beaudine is puzzled.
A speeding Eric Blore as Jamison nearly runs Joan down. A motorcycle policeman arrests her, Jamison, and Warren William as Michael Lanyard, the happily retired gentleman criminal the "Lone Wolf." However, Joan blurts out that she is involved in a murder, so Warren William as Michael Lanyard gives the policeman the slip. He takes Joan back to her apartment, examines it and the body for clues, and provides her with a cover story. As Lanyard and Jamison are leaving, they are spotted and overheard by Beaudine.
When the police arrive, Thurston Hall as Inspector Crane believes Joan, until he learns that the door was not forced and a key found on the dead man fits the lock. He decides to take her in for further questioning, but lets her get properly dressed first. She escapes, as Crane had hoped, as he has arranged to have her followed. She finds Lanyard's address in the telephone directory and heads there.
Beaudine gets there first, convinced that Lanyard and Joan are working together to steal the necklace. Then Joan arrives. After Jamison takes Beaudine into the next room at gunpoint, Lanyard learns about the incriminating key and that Joan got away so easily. When Crane shows up, as Lanyard had expected, Lanyard hides Joan in a secret compartment. After the police and Beaudine leave, Lanyard and Joan go to see Nick, George McKay as Nick, a fence, to ask him to find out if anybody is trying to sell the necklace.
Beaudine comes to Lanyard's place while he is out, but Joan is present. He then recognizes the person who fled the apartment where Rennick was killed from a newspaper photo of the people who were present when Joan received the necklace. After locking Joan and Jamison up, he telephones the person and demands the necklace be brought to his place in 15 minutes. Jamison has, however, turned on a recording device. Bob Penyon and Lanyard arrive just after Beaudine leaves. Lanyard receives a call from Nick, who informs him the necklace had been broken up and sold piecemeal a year before. The recording proves helpful, and Lanyard and Penyon race away. They get Rennick's address from his dry cleaner, who then calls Crane. They find Beaudine dead. From a clue, Lanyard thinks he has solved the case, but Inspector Crane and his men arrive and arrest him. Fortunately for Lanyard, Bob produces a gun. Lanyard telephones Joan and has her inform the suspects that he has figured out who the murderer is and the location of the necklace, in the river beside Joan's apartment. While Bob holds the others prisoner, Lanyard takes Crane to Joan's place. First Peter Van Wyck, then Arthur Trent, and finally Bob's mother drive up. Robert Emmett Keane as Peter Van Wyck, William Forrest as Arthur Trent, and Georgia Caine as Mrs. Penyon drive up. Lanyard eventually identifies Van Wyck as the killer.
