As Dr. Ernest Sovac is ushered to the electric chair to die for the murder of his friend, Professor George Kingsley, he passes his diary to a waiting reporter, who reads the story of his crime:  on Friday the 13th, Kingsley is run over by a car driven by hardened criminal Red Cannon.  The professor suffers a severe concussion, and to save his life, Sovac transplants brain cells from the dying Cannon into Kingsley's skull.

As Kingsley recovers, his wife Margaret discovers that her formerly timid husband now flies into murderous rages.  Reading that Cannon hid half a million dollars in stolen money, Sovac decides to take his old friend to the gangster's haunts in New York, in hopes of jarring Cannon's memory and recovering the money so that he can build a hospital.  Sovac's plan works as Cannon's brain and personality take over Kingsley's body and he undertakes a vendetta against former gang members Louis Devore, William Kane, Frank Miller and Eric Marnay.

On the night after murdering Devore, Kingsley switches back and forth between his own and Cannon's persona but although Sovac realizes that he can no longer control his patient, he continues his experiment.  After Canon influences him to murder Kane, Kingsley is wounded by the police and returns to the hotel to find his wife and Sovac's daughter Jean awaiting him.

Discovering the truth about her father's experiment, Jean demands that they all return home to Newcastle together.  That night, Kingsley retrieves Canon's money and, after killing Miller and Marnay, murders Sunny Rogers, Canon's old girl friend.  The Kingsleys and the Sovacs return to Newcastle, where Kingsley peaceably resumes his teaching chores until the sound of a siren changes him back to Cannon.  Going to the Sovac house, he attacks Jean, forcing Sovac to kill his friend to save his daughter.

As the reporter finishes reading Sovac's diary, the doctor meets his death in the electric chair.