Mary (Kim Hunter) travels to New York to discover the reason for her sister Jacqueline's sudden disappearance. The cosmetics shop that Jacqueline (Jean Brooks) owned has been sold and her rented room is empty, save for a solitary chair and a noose. Suspecting that her sister is under the influence of Satanists, Mary hires a private detective to stakeout the shop at night, but she then discovers that he has been murdered. Dr. Louis Judd (Tom Conway) contacts Mary to explain that he is a psychiatrist and that Jacqueline is under his care because she is mentally ill. But when Jacqueline vanishes again, it becomes clear to Mary that she in the clutches of a satanic cult whose penalty for revealing anything about themselves is death. Six people have already been murdered... will Jacqueline become the seventh victim?