After a detective is assaulted by thugs and placed in an asylum run by Professor Baum (Oscar Beregi), he observes the professor's preoccupation with another patient, the criminal genius Dr. Mabuse the hypnotist (Rudolf Klein-Rogge). When Mabuse's notes are found to be connected with a rash of recent crimes, Commissioner Lohmann (Otto Wernicke) must determine how Mabuse is communicating with the criminals, despite conflicting reports on the doctor's whereabouts, and capture him for good.