Can evil in a human being be cured? Emmanuel Hildern (Peter Cushing), a 19th-century scientist, believes it can, and develops serum from the blood of evil men. While his half-brother James (Christopher Lee), who runs a mental institution, envies Emmanuel's accomplishments, his daughter Penelope (Lorna Heilbron) does not. Born of an insane mother, the girl is injected with her father's serum, slashes a sailor and kills an escaped madman. Sadly, she must then be committed to her uncle's asylum. Undaunted, Emmanuel continues his experiments on a primitive skeleton, inadvertently creating anevil monster when the skeleton comes to life. The trauma of the events that follow drive Emmanuel insane and frees the creature to spread its reign of terror.