A prostitute is murdered in London's West End. Her killer watches film of the act at home. Next day, he is back at the scene with his camera to film the removal of the body. Later, Mark goes to his part-time job, taking 'glamour' photographs above a seedy newsagent. He becomes entranced by the bruise on the face of one of the models.

Helen Stephens celebrates her 21st birthday in the flat she shares with her blind, alcoholic mother. When Mark, whose flat is upstairs, comes home, she invites him to join the party. Uncomfortable, he declines and goes upstairs. He is watching his films when Helen knocks at the door, offering a piece of cake. Shyly, he invites her in. He reveals that the house was his father's and now his.

When he tells her he works at a film studio, Helen asks to see his work. Instead, he shows films taken when he was a boy, showing his unusual relationship with his father. Helen is distressed by the films. Mark explains that his father, a biologist, filmed him constantly, and was particularly interested in fear.

At the film studio, Mark has arranged to film one of the actresses, Viv, for his own personal project, after hours. Explaining he wants to film her fear, he reveals a knife in the tripod of his camera. As he advances towards her, camera running, she screams.

That evening, Mrs Stephens expresses her doubts about Mark: he has 'stealthy' footsteps. Helen, however, is intrigued, and visits him. As a late birthday present, he gives her a dragonfly brooch. She tells him she has written a children's book about a magic camera. Mark is enthused, and offers to take the photographs for the book. They agree to discuss it the next night.

Next day at the studio, Viv's body is discovered in a trunk. Mark films the reaction when the trunk is opened. The police are called, and interview everyone on the set. Mark films what he can. His interview passes without event. He offers to hand over the film in his camera, but the officers decline. As they inspect the crime scene, Mark films from the balcony.

That evening, Mark meets Helen's mother, who quizzes him about the murder at the studio. He denies knowing her. With some difficulty, Helen persuades him to leave his camera behind, and they go out.

They return, happy, but Mark becomes upset when Helen suggests he photograph her. In his rooms, he finds Mrs Stephens. She questions him, but he is evasive. He plays last night's film, but is disappointed by the results - he will have to try again. He turns the camera on Mrs Stephens, but resists the temptation. Unsettled, he asks her to leave. She tells him her instinct tells her he needs help; until then, she asks him not to see Helen.

At the studio, filming goes badly: the lead actress is highly disturbed. Mark talks to a psychiatrist, assisting the police investigation, who knew his father. Mark asks about scoptophilia - voyeurism. He asks if there is a cure, but the psychiatrist is more interested in seeing his father's notebooks. Learning about the conversation, one detective becomes suspicious and follows Mark to the newsagent, where he is doing more of his glamour photography. The shopkeeper leaves Mark with a key, and he goes up to meet the model. Later, Mark leaves alone, aware the detective is still on his tail.

Helen goes to Mark's room. Finding him out, she switches on the projector. She is appalled. Mark returns. He admits to the murders, but she wants to know more. He plays her his father's sound recordings: the whole house was wired, and still is. Meanwhile, the model's corpse has been found in the newsagents. Mark tells Helen that he has found a way to photograph fear: with a mirror attached to his camera, he forces his victim's to witness their own fear as they die. As the police approach and the horrified Helen looks on, he prepares his final scene, which he has rehearsed countless times. The police enter to find Mark already dead; Helen lies weeping over his body.