The story involves an attempt to keep alive the brain of millionaire megalomaniac W.H. Donovan after an otherwise fatal plane crash. The brain has other ideas and begins to possess people.
Plot
Dr. Patrick Cory and his wife Janice live in a mountain retreat where Cory attempts to keep a monkey's brain alive after having it removed from the monkey's skull.
The private plane of businessman Warren Donovan crashes near Cory's cabin, and rescuers request Cory's help. Donovan is seriously injured and is taken to Cory's cabin. There, his heart stops, so Cory takes the businessman's brain for experimentation. Cory manages to keep the brain alive in an electrified saline solution. After writing messages in Donovan's handwriting while he is sleeping, Cory believes Donovan's consciousness still survives and he attempts to communicate with the brain.
Gradually, Cory begins to exhibit Donovan's personality traits such as smoking cigars, using ruthless personal manipulation, and walking with a limp. Janice and Frank Schratt, Cory's friend and assistant, suspect that Donovan's consciousness is using telepathicmind control to overpower Cory's free will. In the meantime, news photographer Yocum discovers that Cory has illegally stolen Donovan's brain and demands money to keep the secret.
Donovan's brain grows increasingly powerful, using Cory to collect a financial fortune and taking control of Yocum's mind and forcing him into a fatal car crash. After realizing that Donovan can control only one person at a time, Janice and Frank plot to destroy the brain. However, Frank's plan goes wrong when Donovan forces Frank to shoot himself. Ultimately, lightning strikes the Cory home and a fire breaks out, burning Donovan's brain and bringing an end to the horror. Frank survives, and Cory willingly goes to accept the consequences for his actions, while Janice reminds the authorities that it was Patrick's idea, in a moment of freedom from Donovan's brain, to connect the ranch's lightning rod to the power supply, which resulted in the fire.