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.

Dr. Cordelier, living in a suburb of Paris, withdraws from society to pursue research into the functioning of the human brain. His lifelong friend, Maître Joly, becomes concerned when Cordelier draws up a will that bequeaths his entire estate to a stranger, Monsieur Opale; he cannot understand why Cordelier defends him, considering Opale attacks women and children. After a colleague is killed, Joly confronts Cordelier and discovers the truth behind his friend's behavior.

Dr. Mabuse has been locked away for years in an insane asylum. Suddenly crimes start occurring and no one knows the culprit. A detective goes to visit the Doctor to find some sort of clues to the robberies.

When the grandmother decides that it is time to put the family affairs in order, the fights begin to inherit the house. (A sequel to Grandma's Wedding.)

A metaphysical mystery involving a university student's camera getting stolen, and the thief then committing suicide.