William Sebastian (Robert Culp) is a former criminologist who now studies the occult to explain the problem of human evil. He has been cursed on one of his adventures by the demon Asmodeus, leaving him in constant need of medical attention. He summons an old colleague, Dr. "Ham" Hamilton (Gig Young) to his home to help him with a case involving the Cyon family. Dr. Hamilton does not believe in the occult and thinks that Sebastian and his housekeeper Lilith (Majel Barrett) are playing tricks on him when he witnesses unusual events.