Mark Lewis (Carl Boehm) meets a prostitute, covertly filming her with a camera hidden under his coat. Shown from the point-of-view of the camera viewfinder, tension builds as he follows the woman into her house, murders her and later watches the film in his den as the credits roll on the screen.
Lewis is a member of a film crew who aspires to become a filmmaker himself. He works part-time photographing soft-porn pin-up pictures of women, sold under the counter. He is a shy, reclusive young man who hardly ever socializes outside of his workplace. He lives in his (dead) father's house, leasing part of it and acting as the landlord, while posing as a tenant himself. Mark is fascinated by the boisterous family living downstairs, and especially by Helen (Anna Massey), a sweet-natured young woman who befriends him out of pity.
Mark reveals to Helen through home movies taken by his father (played by director Powell in a cameo) that, as a child, he was used as a guinea pig for his father's psychological experiments in fear and the nervous system. Mark's father would study his son's reaction to various stimuli, such as lizards he put on his bed and would film the boy in all sorts of situations, even going as far as recording his son's reactions as he sat with his mother on her deathbed. He kept his son under constant watch and even wired all the rooms so that he could spy on him. The father's studies made his reputation as a psychologist.
Mark arranges with Vivian (Moira Shearer), a stand-in at the studio, to make a film after the set is closed; he then kills her and stuffs her into a prop trunk. The body is discovered later by the horrified film crew. The police link the two murders and notice that each victim died with a look of utter terror on her face. They interview everyone on the set and become suspicious of Mark, who has his camera always running, always recording and who claims that he is making a documentary.
A psychiatrist, called to the set to console the upset star of the movie, chats with Mark and tells him that he is familiar with his father's work. The psychiatrist relates the details of the conversation to the police, noting that Mark has "his father's eyes".
Mark is tailed by the police who follow him to the building where he takes photographs of the pin-up model Milly (Pamela Green). Two versions of this scene were shot. The more risqué version is credited as being the first female nude scene in a major British feature  (although even on the racier version, Milly only exposes one breast for a few seconds). Mark kills Milly and then returns home.
Helen, who is curious about Mark's films, finally runs one of them. She becomes visibly upset and frightened when he catches her. Mark reveals that he makes the movies so that he can capture the fear of his victims. He has mounted a round mirror atop his camera, so that he can capture the reactions of his victims as they see their impending deaths.
The police arrive and Mark realizes he is cornered. As he had planned from the very beginning, he impales himself with a knife attached to one of the camera's tripod legs, killing himself the same way he dispatched his victims, and with the camera running, providing the finale for his documentary.