In London, young women go missing after responding to mysterious advertisements in the newspapers. Scotland Yard enlists the help of Sandra Carpenter, an American dance-hall hostess, to lure the killer out of hiding by answering the ads. Sandra encounters a variety of eccentric characters, including a suspicious dress designer (Boris Karloff) who turns out to be harmless. Ultimately, she uncovers a gang of white slavers led by Dr. Moryani (Joseph Calleia) who is believed to be responsible for the murders. However, her fiancé, Robert Fleming (George Sanders), a nightclub owner, is falsely accused and convicted of the murders. Sandra risks her life to clear his name and reveal his business partner (Sir Cedric Hardwicke) as the real killer.