A thief breaks into an underground laboratory. A masked man decapitates a doctor, and carrying the head, enters the lab and battles the thief, who flees in horror. Removing his mask, the man reveals himself as Baron Victor Frankenstein (Peter Cushing). The thief is so scared he reports the severed head to the police station, speaking to Inspector Frisch (Thorley Walters). Frankenstein, calling himself Mr. Fenner, rents a room at the boarding house of Anna Spengler (Veronica Carlson), whose fiancé, Karl Holst, (Simon Ward) is a doctor at the asylum where Frankenstein’s assistant Doctor Frederick Brandt (George Pravda) was committed.
Frankenstein learns that Karl has been stealing drugs to support his mother-in-law and blackmails Karl into helping him kidnap Brandt so he can get his hands on the doctor’s secret formula. While stealing equipment, Karl and Frankenstein are caught. Karl panics and stabs the guard. Now, Frankenstein has even more power over Karl and he and Anna help Frankenstein to kidnap Brandt. They take him to a lab they have built in a basement. Karl confesses to Anna, hoping she will not be implicated in his crimes, but she stands by him.
Brandt has a heart attack, so the team kidnaps the administrator of the asylum, Professor Richter (Freddie Jones) to transplant Brandt’s brain into his body. Frankenstein rapes Anna. The next day, the brain is transferred and they bury Brandt’s body in the garden. Brandt’s wife Ella (Maxine Audley) recognizes Frankenstein on the street and confronts him. Getting no satisfaction from him, she goes to Frisch. The creature recovers and the team relocates to a deserted mansion. The creature awakes and sees himself in a mirror. He scares Anna and she stabs him, but he escapes. Frankenstein, finding the creature gone, stabs Anna. The creature returns to his home but is not recognized by his wife. He prepares to burn down the house. Frankenstein arrives and tries to flee with his papers, but the creature drags him into the fire.
The film was directed by Terence Fisher from a screenplay by Bert Bratt, the fifth Frankenstein movie for Hammer Films. Frankenstein was even more insane than usual in this one. The rape scene was added by Hammer executive James Carreras at the demand of the American distributors, who wanted more sex in the story, but everyone hated it. Peter Cushing apologized to Veronica Carlson for the rape scene and took her out to dinner with his wife. Classy, as always. He was supposed to tear off her top, which was all right with her, but Cushing refused outright. What they end up with was a scene that was not titillating but terrifyingly revealed the madness of Frankenstein and his mania to control everyone around him in the interest of his precious creature’s survival. Inspector Fritsch was added too, for comic relief, and everyone hated that too.