Capricious small-town girl Juliette and barge captain Jean marry after a whirlwind courtship, and she comes to live aboard his boat, L'Atalante. As they make their way down the Seine, Jean grows weary of Juliette's flirtations with his all-male crew, and Juliette longs to escape the monotony of the boat and experience the excitement of a big city. When she steals away to Paris by herself, her husband begins to think their marriage was a mistake.

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.