There had been Hollywood films about satanism going back to the silents, but this was new in depicting a devil cult in contemporary New York among ordinary people doing unremarkable jobs. Cuts imposed by RKO left problems with plot continuity, but it hardly matters. This is mainly a work of atmosphere and psychological anxiety.
It shows the apprehensive journey of a young woman (Kim Hunter) into sexual maturity. The film subtly suggests that what lies in the darkness and behind doors is her unease over her erotic awakening. Her quest is to find her sister (Jean Brooks) who joined the satanists, but broke their code of silence. And so must die, like six others before.
There are some brilliantly innovative moments of suspense, most potently a scene on the subway where the girl witnesses a man she has just seen murdered held up between two heavies, as if they were all drunk. There is also a very interesting shower scene which may have influenced Psycho.
This is a film of dense emotional dread, of despair. The lost sister is portrayed as a figure of extreme moral emptiness, without will. Her last scene with a dying neighbour (Elizabeth Russell) is astonishing. It is an intensely pessimistic film which offers little hope. It is unique in '40s Hollywood, and an intelligent, audacious horror landmark.
Reviews, such as the first one here, had led me to have high hopes of this. In the event, as others have found, it turned out to be a series of often-enjoyable scenes which did not cohere. Perhaps the studio made cuts at the time. This said, it is worth watching at little more than an hour.