Dr. X, a shadowy figure and leader of a drug trafficking organization, is unknown even to his own henchmen. Anyone who attempts to uncover his identity is killed. Secret Service agent Jack Hart (played by Jack Holt) infiltrates the gang by gaining the trust of Henderson (played by Boris Karloff), a member of the organization. Through Henderson, Hart meets Arnold (played by Claude King), a superior in the gang, and joins the group. However, things become complicated when he meets Arnold's daughter, Julie (played by Constance Cummings), and falls in love with her.
Arnold is later killed by Dr. X for talking too much. Julie is kidnapped and Hart traces her to a private hospital run by Dr. Steiner (played by Edward Van Sloan). There, Hart's cover is blown and he is strapped to an operating table. Dr. Steiner reveals himself to be Dr. X, and is about to perform a fatal operation on Hart, but he is shot by Julie before he can do so.