The Doctor Patrick Cory and his friend the Doctor Frank Schratt lead experiences on the brain. For the first time they succeed in preserving alive the brain of a monkey out its body.
When the private plane of millionaire Warren H. Donovan crushes near their laboratory at Green Valley, the former, between life and death, is driven there in emergency. Unfortunately the rich man dies on the table of operation. Despite the disapproval of Frank and his assistant and wife Janice Cory, Patrick extracts in the greatest illegality the brain of Donovan still alive, seeing in this act the unique chance to study the human brain and the process of thoughts.
The death of the millionaire attracts a quantity of journalist and among them, the photographer Herbie Yocum insists to get some pictures of the table of operation on which Donovan lost his life. But the former profits an instant of inattention of the two physicians to take a cliche of the brain in the laboratory.
Would it be the electrical excitation provided to the brain or another natural or strange phenomenon, the fact is that the brain enters in telepathic contact with Patrick Cory. In its nutrient liquid, the brain of Donovan enlarges each day a bit more and seizes little by little the mind of the scientist who has now reached a double personality. When Donovan's brain is lulled, Patrick Cory is the scientist who loves his wife, but when the brain is awaken, it is Warren H. Donovan who lives in the body of this last. The mind of the irascible millionaire with a strong personality takes over and uses Patrick to run all sorts of financial businesses.
While Donovan continues his strange financial scheme, Patrick Cory cannot hold it anymore. Tired no to be able to control his thoughts, he throws under the wheels of a truck to liberate himself from the influence exerted by the brain. More fear that pain. But Janice understands very fast that two persons live by the body of her husband. Wanting to help him to get rid of his pain, Frank attempts to destroy the brain by unplugging the electrical feeding that maintains it alive. But Donovan's brain perceives the situation and without Janice intervention, Patrick would have succedeed in killing his friend Franck Schratt...
Spoiler ! the end of the story is revealed below…
Donovan wants to dominate the international financial scene. In Patrick's body, he insures that an accident -- such as the one that occurred to the photographer Herbie Yocum, who was blackmailling Cory with the photos illicitly shot in his Green Valley laboratory -- will arrive to all who will cross his path. But his wife Janice and his friend Frank are on his path.
Frank knows that the brain can concentrate only on a single thing at a time. With Janice, they elaborate a plan to get rid of the brain and to save Patrick. But their plan fails and Frank is injured. While all seems lost, the thunder shots the laboratory of the scientists and destroys the brain. Patrick Cory sees finally liberated from the ascendancy of Donovan. Having to reply for his acts, Patrick possesses the evidences of his innocence: a recording proving that it has connected himself the lightning conductor of the laboratory on theinstallation of Donovan's brain.
Adapted from "Donovan's Brain", the novel published in 1942 by the german emigrated Curt Siodmak, this movie represents its best adaptation. The novel has equally inspired the movie "The Lady and the Monster" by George Sherman in 1944 and "Vengeance" also known as "The Brain" directed by Freddie Francis ("Evil of Frankenstein", "Day of the Triffids") in 1962.