After young Mary Gibson (Kim Hunter) discovers that her older sister Jacqueline (Jean Brooks) has disappeared, she leaves her boarding school and heads to New York City to track down her sibling. Arriving at Jacqueline's apartment, Mary finds ominous signs of trouble, with her sister nowhere in sight. Aided by psychiatrist Dr. Louis Judd (Tom Conway) and others, Mary gets drawn deeper into the mystery, and evidence begins to point to the involvement of a sinister cult.