I distinctly remember being truly frightened when I first experienced this film back in the early 1970s. The narrative centers on young women who are discovered murdered, and what makes these offenses particularly disturbing is the expression of pure dread visible on their faces. This detail also adds to the confusion for the Scotland Yard detective named "Gregg" (played by Jack Watson). Simultaneously, actress "Vivian" (Moira Shearer) is working on her latest movie, and on set is "Mark" (Karlheinz Böhm), who has a fascination with cameras and leases out the lower level of his large house to a group of young people. Typically withdrawn, he begins to connect with "Helen" (Anna Massey), and soon a connection forms as she shows interest in both him and his films. Following an incident at the studio, the police investigation initially involves the film crew, and the story gradually reveals itself in a shadowy and threatening manner. Böhm and Massey give strong performances, and under Michael Powell and Otto Heller’s artistic guidance and cinematography, they present us with an intimate and deliberately paced narrative that unfolds slowly but builds to a powerful effect. The conversations are surprisingly impactful; there's something almost innocent about "Mark" and a maternal quality to "Massey," which highlights the underlying psychology of the story. On a stormy evening with rain against the glass, this is as good as film can be.

Source on TMDB