Mild-mannered College professor George Kingsley (Stanley Ridges) is caught in a bloody gangster shoot out and is gravely injured. Kingsley's close friend Dr. Ernest Sovac (Boris Karloff) performs an emergency operation replacing the damaged section of Kingsley's brain with that of injured gangster Red Cannon (Bela Lugosi). But while the operation is a success, Kingsley experiences disastrous 'Jekyll and Hyde' side effects, occasionally taking on the more brutal aspects of Cannon's personality. While under one of his 'spells', Kingsley reveals the existence of a stash of stolen money. Struggling to control his greed, Sovac hypnotizes Kingsley bringing out his alter ego, hoping to get information on the location of the money. But hypnosis brings Kingsley closer to madness, with his personalities alternating between docile professor and brutal gangster, out for revenge.