After having a detective is attacked by thugs and placed in an asylum run by Professor Baum, he observes the professor of the preoccupation with another patient, the legal genius Dr. Mabuse the hypnotist. When the notes of Mabuse have been seen to be connected using a rash of recent crimes, Commissioner Lohmann catch him to get good, and must determine Mabuse is communicating with the criminals, even though conflicting reports on the whereabouts of the doctor.