University professor George Kingsley is struck by gangsters while crossing the street, leaving him with brain damage and one of the gangsters, Cannon, paralyzed. Kingsley's friend Dr. Sovac attends to both men, and when Cannon offers him a reward for aiding his recovery, Kovac transplants part of Cannon's brain into the dying Kingsley's skull, creating a dual personality.

• Universal Pictures
• Stanley Ridges
• Boris Karloff
• Bela Lugosi
• Anne Nagel
• Anne Gwynne
• Virginia Brissac
• Edmund MacDonald
• Paul Fix