In London, Mark Lewis, Karlheinz Böhm, a shy, reclusive focus puller on a film crew, covertly films Dora, Brenda Bruce, a prostitute, with a camera hidden under his coat. He follows her into her flat, murders her with a blade concealed in one leg of his tripod, and later watches the film in his darkroom. The following morning, posing as a reporter, he films the police removing Dora's corpse from her home.
Lewis is a member of a film crew who aspires to become a filmmaker himself. He also works part-time photographing soft-porn pin-up pictures of women, sold under the counter. He is a shy, reclusive young man who hardly socialises outside of his workplace. He lives in the house of his late father, renting most of it via an agent while posing as a tenant himself. Helen Stephens, a sweet-natured young woman who lives with her blind mother in the flat below his, befriends him out of curiosity after he is discovered spying on her 21st birthday party.
Mark reveals to Helen, through home films taken by his father, Michael Powell (A.N. Lewis), that, as a child, he was used as a guinea pig for his father's studies on fear in children. Mark's father would study his son's reaction to various stimuli, such as large lizards he put on his bed, and would film the boy in all sorts of situations, even recording Mark's reactions at his mother's deathbed. (He married another woman suspiciously soon afterwards.) His studies enhanced his reputation as a renowned 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 during shooting by Pauline, Shirley Anne Field, a cast member who has already antagonised the fussy director by acting a faint badly, and then actually fainting from exhaustion at the many retakes. 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, including Mark, who always keeps his camera running, claiming that he is making a documentary.
Helen goes out to dinner with Mark, persuading him to leave his camera behind for once, and briefly kisses him once they return. Her mother finds his behavior peculiar, being aware, despite her blindness, that Mark often looks through Helen's window. She is waiting inside Mark's flat after his evening out with her daughter. Unable to wait until she leaves due to his compulsion, he begins screening his latest snuff film with her still in the room. She senses how emotionally disturbed he is and threatens to move, but Mark reassures her that he will never photograph or film Helen.
A psychiatrist is called to counsel Pauline. He chats with Mark and 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 to the newsagents, where he takes photographs of pin-up model Milly, Pamela Green. Shortly afterwards, it emerges that Mark has killed Milly.
Helen, who is curious about Mark's films, finally runs one of them. She becomes upset and then frightened when he catches her. Mark reveals that his father not only filmed his experiments on him, but recorded them and has wired the house, and he plays a recording of her with her boyfriend. Mark makes his films so that he can capture the fear of his victims. He has mounted a mirror on his camera so that he can film his victims' reactions as they see their own impending deaths. He points the tripod's blade towards Helen's throat and we see her reaction distorted in the mirror, but he cannot kill her.
The police approach and Mark hears their sirens. As he planned from the very beginning, he impales himself on the knife with the camera running, providing the finale for his documentary. Helen cries over Mark's dead body as the police enter the room. As the scene fades, we hear a recording of his father calling him a good boy.
