While investigating the murder of nightclub owner Albert Simoni, Inspector Vallois of the Paris police becomes emotionally involved with the victim's German mistress Lucky Friedel, a would-be singer who takes dope. Hoping to prove her innocence, Vallois tries to induce Lucky to tell him all she knows, but she refuses to cooperate. Eventually, however, she leads him to Thérèse Marken, a clever woman pharmacist who also knew Simoni. Vallois' slow but methodical handling of the case so irritates the police commissioner that he decides to take over and arrest Lucky for drug addiction as well as complicity in Simoni's murder. Before he can do so, however, Lucky disappears. Vallois finds her, heavily doped, in Thérèse's home. Confronted by Vallois, Thérèse admits to having killed Simoni when he tired of her affections. Lucky witnessed the crime and used her knowledge of it to blackmail Thérèse into supplying her with morphine. After telling headquarters where they can find the killer, Vallois takes Lucky to a sanitarium, promising to return for her once she has been cured.
