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 (Dave Prowse) is a compilation of body parts taken from the dying inmates in the asylum, many of whom Frankenstein euthanises for his purpose. Helder is uneasy with the Barons utilising of the inmates to further his research and tells the Baron his feelings, but the Baron confides in Helder a secret. His lovely young assistant Sarah (Madeline Smith) is 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.