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
body and he undertakes a vendetta against former gang members
Louis Devore, William Kane, Frank Miller and Eric Marnay.
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 (Anne Gwynne) 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.