Professor Marcilly is a famed brain specialist who, following in-depth and extended research work, is now able to perform brain transplant surgery. One day, he finds himself in the presence of a young car accident victim for whom the only hope of survival would be a brain transplant. Marcilly, who has a heart condition and is terminally ill, decides to become the "donor". The operation is a success. But who is actually the patient discharged from the hospital: a young fellow with the brain of a young man or an old man in the body of a young one?