Mary Gibson leaves school to go to New York and search for her missing sister Jacqueline. Upon arriving in the city, Mary visits La Sagasse, the cosmetics store owned by her sister, and learns that Jacqueline has sold the business to Mrs. Redi. Mrs. Redi claims no knowledge of Jacqueline's whereabouts, but Frances, an employee of the company, tells Mary that she recently saw her sister at a restaurant in Greenwich Village. At the restaurant, Mary meets the proprietors, Mr. and Mrs. Romari, who remember renting an upstairs room to the beautiful and mysterious Jacqueline, who never moved into the room. Mary convinces the Romaris to unlock the room, and upon opening the door, they see a noose dangling from the ceiling. Mary is consoled in her search by Jason Hoag, a failed poet, who advises her to file a report at the missing person's office. At the police station, Mary meets private investigator Irving August, who offers to find her sister for fifty dollars. When Mary finally reveals that she does not have the money, August is about to drop the case when a man warns him to forget about finding Jacqueline. Intrigued, August pulls her file. Mary, meanwhile, visits the morgue, where officials suggest she consult with attorney Gregory Ward. At his office, Gregory admits that he is in love with Jacqueline and explains that the noose is part of her fascination with death. After dining with Gregory, Mary returns to her hotel and finds August waiting for her. When August reports that he discovered a locked room at La Sagasse, Mary insists upon investigating. Mary waits in a deserted corridor in the store, while August enters the locked room. The detective soon stumbles out and collapses on the floor, the victim of a fatal stab wound. Mary races out and boards a subway train, where she later sees two men supporting August's limp body. Although Gregory remains skeptical about Mary's murder story, he finds her a job as a kindergarten teacher, and she moves into a room at the Romaris'. Soon after, Louis Judd, a psychiatrist, visits Gregory to ask him for money to support Jacqueline. When Gregory demands to see her, Judd warns him that Jacqueline is in danger of losing her sanity. Alerted to Mary's presence by Gregory, Judd offers to take her to Jacqueline's apartment, but they find it empty. Judd leaves to search for Jacqueline, who soon after, opens the apartment door, sees Mary, closes the door and disappears. Mary goes to look for her sister, and upon returning to the apartment, she is confronted by two men who identify themselves as private detectives hired by Jacqueline's husband, Gregory Ward. That night over dinner, Gregory explains that he kept his marriage a secret to protect Jacqueline. Jason then joins them at their table and promises Mary that he will find Jacqueline. Taking Mary and Gregory to a soiree at the apartment of Natalie Cortez, Jason questions Judd, one of Natalie's guests, about Jacqueline. The next day, Jason goes to the library and discovers that both Judd and Mrs. Redi had checked out a book about the Palladists, a sect of devil worshippers. Suspicious, Jason sends Mary to the cosmetics store to find out more about Mrs. Redi. At the shop, Mary questions Frances and discovers that the firm's new trademark is the Palladist diagram. Mary returns home and is showering when the shadowy figure of Mrs. Redi appears on the other side of the shower curtain. After informing Mary that Jacqueline killed August, Mrs. Redi advises her to return to school. Back at Natalie's apartment, a meeting of the Palladists has convened. Insisting that Jacqueline must die because she betrayed the society by visiting Judd, a psychiatrist, Mrs. Redi dispatches Lee and Durk, the men from the subway, to find her. Mary, taking Mrs. Redi's advice, visits Jason in his room to bid him farewell. When Mary explains that she is returning to school because her sister is a murderer, Jason, whose love for Mary has prompted him to begin writing poetry again, insists that she consult with Gregory first. When Gregory advocates that Jacqueline turn herself in to the police, Jason asks Judd to locate her. Judd brings Jacqueline to Jason's room, where she tells them that she sought help from Judd to break with the Palladists who, feeling betrayed, locked her in a room at La Sagasse. Tormented by fear and isolation, she lashed out at August with scissors when he entered her room. The next morning, Mary goes to school, where Romari phones to tell her that Jacqueline left with two men. The men take Jacqueline to Natalie's apartment, where the Palladists condemn her to death and demand that she drink a chalice filled with poison. As the hours tick by, Jacqueline's resolve ebbs and she reaches for the poison, but Frances screams and knocks the glass from her hand. After releasing Jacqueline into the night, Mrs. Redi sends an assassin after her. Escaping the assassin's knife, Jacqueline runs back to her room at the Romaris', where she meets a deranged woman in the hallway. The woman, who lives across the hall, announces that she is dying, but vows to experience life before she perishes. While Mary and Gregory await Jacqueline's return, Gregory tells Mary that he has fallen in love with her, and Mary admits that she also loves him, but could never take him away from her sister. Later that night, the deranged woman, attired in an evening gown, leaves her room, and as she passes Jacqueline's door, she hears the sound of a rope snapping.