Last of the Hammer Frankenstein films, this one deals with the Baron hiding out in an insane asylum, so that he may continue his experiments with reanimating the dead, along with inmate Dr. Helder, who has been institutionalized for conducting such experiments.
A body snatchers caught by a police sergeant and he snitches the name and address of his client, Dr. Simon Helder. The doctor is arrested and accused of sorcery, and sent to a psychiatric institution. There he meets Dr. Carl Victor, a.k.a. Baron Victor Frankenstein, who is presumed dead but actually he is alive and secretly continuing his experiments reanimating the dead. Dr. Helder worships Frankenstein and has studied his works and he becomes his assistant together with the dumb Sarah. One day, Dr. Helder discovers Dr. Frankenstein's secret laboratory and accidentally releases a Monster in the institution, bringing panic to the inmates and staff.
Baron Frankenstein works with a mental patient to reanimate the dead.
- Simon Helder (Shane Briant) spends his nights piecing together body parts from cadavers for his research but is brought before a judge and sentenced to 5 years in an insane asylum where he is tortured by the orderlies. Little does Helder know but Baron Frankenstein (Peter Cushing) has been hiding out in the asylum and when he discovers the young doctor he enlists him as an assistant to help in the tending of the medical needs of the other inmates but ultimately to help with the creation of a new monster he is bringing to life. The monster (David Prowse) is a compilation of body parts taken from the dying inmates in the asylum, many of whom Frankenstein euthanizes for his purpose. Helder is uneasy with the Barons utilizing of the inmates to further his research and tells the Baron his feelings, but the Baron confides in Helder a secret that is the cause of his lovely young assistant Sarah (Madeline Smith) a mute who is the daughter of the lecherous Asylum Director (John Stratton). As the experiments progress and the monster is created it becomes apparent that there is an incompatibility of body parts, driving the monster homicidally insane and aggressively vicious.