As her friend suffers from a violent appendicitis attack, Ellen takes her to a London hospital in the dead of night. Despite the night nurse's reticence, her friend is taken into care. Two surgeons are on call this night and one of them performs the operation. But Ellen's relief is short-lived for, while leaving the hospital, she surprises an alarming conversation about what the hospital's real activities are. Feeling threatened, she takes refuge in the room of a patient and to her horror she realizes the man is dead! Worse, he has just been operated on by one of the two surgeons on duty. Obviously one of them is a manic killer but which one?