Like Hitchcock's more famous Psycho (released six months after) Peeping Tom is the story of a psychopathic killer. The titular character Mark is a photography assistant in a movie studio. After a strange childhood in which he has constantly been recorded and documented as the subject of his neurologist father's experiments Mark has developed an obsession for the camera lens and what it captures. Mark is also a killer that stabs his victims and records their dying moments on film.
The story is told mostly from Mark's point of view and one of the most striking aspects of the screenplay is the level of sympathy his character is given, probably also the cause of its initial downfall with the censors and the critics. There is no shying from Mark's more despicable activities. None of his victims, even when they are "two-quid" whores, are shown to be “asking for it”. But Mark is also revealed as a painfully shy loner, a victim of his bizarre upbringing. When Helen the girl-next-door makes a friendly overture, his response is one of almost child-like happiness and gratitude. Carl Boehm in the lead role does a spectacularly convincing job, being simultaneously creepy and pitiable. Even his slight German accent works to underline Mark's distance from his surroundings.
Where the film again succeeds is in the brilliant material for the supporting characters, the best example being Helen's mother, blind and alcoholic but steely-nerved, and ironically the only person to see through Mark's dupe. She also gets some of the film's best lines; to Helen's defense of Mark saying “He's just shy”, she counters royally with “His footsteps aren't. They're stealthy.”
Powell develops a compelling visual style for the film, with deliberately lurid colors (including a prominent red motif) and POV shots that constantly reiterate the film's discomfiting voyeuristic themes. The use of a jazz-flavored dance score to build tension before one of the murder set-pieces is executed with a panache Hitchcock would have envied.
To my mind, this film takes a more difficult line than Psycho did. That it manages to walk without stumbling (that too by a maker who had never before handled a film of this flavor), makes it a more significant achievement than the latter film. Your opinion may well differ, but there is no dispute that Peeping Tom is a film that MUST be seen and savored by fans of the psychological horror genre.