The brain surgeon in question is Sir Joel Cadman, played by Basil Rathbone. He rescues his former student, Dr. Gordon Ramsay, played by Herbert Rudley, from execution and brings him to his remote abbey to continue his research on the human brain. Cadman has been trying to perfect the use of a drug that can induce a temporary state of suspended animation, or "black sleep", which he believes will allow him to perform brain surgery without the risk of the patient dying.
Ramsay is initially hesitant to assist Cadman, but eventually becomes captivated by the potential scientific breakthroughs that could come from mapping the functions of the brain. He is aided in his work by a faithful servant, Mungo (played by Bela Lugosi), who is grateful to Cadman for saving his life and now serves him with unquestioning loyalty.
The subjects used in Cadman's experiments are criminals who have been condemned to death, and Ramsay is disturbed by the inhumanity of treating them as little more than laboratory animals. He is particularly drawn to the daughter of one of the subjects, Laurie (played by Patricia Blake), who has been visiting her father in prison and is desperate to save him.
As Ramsay begins to question Cadman's methods and motives, he discovers that the "black sleep" drug has had unexpected side effects on the subjects. Some have emerged from their comas with heightened psychic abilities, while others have become dangerous, violent monsters.
Ramsay enlists Laurie's help in discovering the truth about what is happening in the abbey, and they eventually uncover a dungeon-like cellar where the surviving subjects are being kept in a state of perpetual misery. Ramsay confronts Cadman about his unethical behavior, but ultimately decides to help him cure the subjects of their degradation.
The Black Sleep was released in 1956 and was directed by Reginald Le Borg. The film was notable for its impressive cast of horror icons, including Rathbone, Lugosi, and Lon Chaney Jr. as the brutish but sympathetic henchman of Cadman. The film is now considered a cult classic among horror fans.