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Fisher, an ex-detective, decides to take one final case when a mysterious serial killer claims the lives of several young girls. Fisher, unable to find the culprit, turns to Osbourne, a writer who was once respected for his contributions to the field of criminology. Fisher begins to use Osbourne's technique, which involves empathizing with serial killers; however, as the detective becomes increasingly engrossed in this method, things take a disturbing turn.

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.