Hans, Karl and Rudi haven't had much luck in their lives so far. One day, Hans receives a letter from his aunt Gudula. She demands that he reanimate her little theatre "Euterpia" in Büllesheim and threatens to disinherit him if he doesn't manage to outcompete her competitor, Wengen, from the theatre "Thalia". In addition, niece Hilde is sent to Büllesheim to support Hans.

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