Mary, a young woman at Miss Highcliff's boarding school, finds out that her sister Jacqueline, who is her only relative, has gone missing and has not paid her tuition in months. The school officials tell her she can only stay on if she works for the school, to pay her tuition.
Mary decides to leave school and try to find her sister. She returns to New York City, and finds that her sister had sold her cosmetics business eight months earlier. She locates the apartment Jacqueline was renting, and finds the only things in the room are a chair and a noose hanging from the ceiling. Knowing that her sister has never feared death only makes Mary more anxious and determined to find her.
Her investigation leads her to Jacqueline's secret husband Gregory Ward, a failed poet, and a mysterious psychiatrist, Dr. Judd. A private detective helps her in her investigation, but is killed.
Dr. Judd helps her locate the sister, who turns out to be hiding from a Satanic cult her co-workers were part of called the Palladists. During the investigation, Ward falls in love with Mary, his sister-in-law. Meanwhile, Jacqueline is kidnapped by the cult members who insist that she kill herself, because the rules of the cult state that anyone who talks about the cult must die. She would be the seventh person condemned under these rules, hence the film's title.