After having a detective is attacked by thugs and put into an asylum run by Professor Baum, he observes the professor's preoccupation with yet another patient, the criminal genius Dr. Mabuse the hypnotist. Commissioner Lohmann capture him to get good, and must determine just how Mabuse is conveying with the offenders, despite conflicting reports on the where abouts of the doctor when Mabuse's notes have been seen to be connected with a rash of crimes.