The film follows the story of Dr. John Pierre (played by Vincent Ball) who is wrongly convicted of a crime and sent to a prison hospital for the criminally insane. Dr. Pierre is a talented surgeon, and Dr. Callistratus (played by Donald Wolfit) sees potential in him as a test subject.
Dr. Callistratus is a former Nazi doctor who has been executed for his heinous crimes. However, he has been secretly resurrected through a heart transplant by his loyal assistant Carl (played by Victor Maddern). Dr. Callistratus is now obsessed with continuing his research into blood deficiency and immortality.
Dr. Pierre soon realizes that he is not in an ordinary prison hospital but is instead a captive in Dr. Callistratus's laboratory. Dr. Callistratus and Carl have been conducting gruesome experiments on the inmates to maintain their own immortality. Dr. Pierre is horrified to discover that they have been draining the blood from the inmates and giving them transfusions of animal blood, causing their bodies to wither away.
As Dr. Pierre tries to escape, he meets a beautiful nurse named Madeleine (played by Barbara Shelley), who is also being held captive by Dr. Callistratus. Together they plan their escape, but Dr. Callistratus is always one step ahead.
The film culminates in a thrilling showdown between Dr. Pierre and Dr. Callistratus, with some violent and shocking twists along the way. The film's chilling atmosphere, strong performances, and suspenseful plot make it a classic of the horror genre.