In New York City, after appearing on the television program Peeping Toms , model Danielle Breton runs into the show’s contestant, African-American Philip Woode, an advertising manager. Laughing about their respective prizes from the show, a cutlery set for her and an evening of dining and dancing at the African Room for Philip, Danielle suggests they use his coupon for dinner. Philip agrees and the couple goes to the club where, over drinks, Danielle relates that she is from Quebec. As Danielle grows increasingly tipsy, a stranger introducing himself as her husband Emil Breton, appears and insists Danielle accompany him home, but she refuses, telling Philip they are divorced. When Emil persists, Philip summons the club’s bouncers who escort Emil away. Philip then offers to take Danielle home and is surprised when she reveals that she lives on Staten Island. At her building, Danielle invites Philip inside to her sparsely furnished apartment, to which she has only recently moved. Peering out a window, Philip notices Emil standing in the street below. When Danielle grows angry over Emil’s presence, Philip offers to drive away, park around the corner and return after Emil departs. After the suggestion goes as planned, Philip returns and spends the night with Danielle on the pullout sofa bed. The next morning, Philip awakens to overhear a brief argument in French from the bedroom between Danielle and another woman. A few moments later, Danielle apologizes for awakening Philip, then reveals that her twin sister Dominique is visiting to celebrate their birthday that day and is very sensitive about strangers. When Philip offers to leave, Danielle pleads with him to fill a prescription as she feels unwell. Returning from the pharmacy a little later, Philip impulsively stops at a bakery to purchase a birthday cake. Meanwhile, Emil telephones Danielle, who admits to feeling ill and asks him to come to the apartment. Overcome by pain moments later, Danielle collapses on the bathroom floor. After having Danielle's and Dominique’s names written on the cake, Philip returns to the apartment and seeing the woman he thinks is Danielle under the covers of the pull-out bed, lights candles on the cake and, retrieving the knife from the prize cutlery set, brings the cake to the bed as a surprise. Groggily, the figure on the bed reaches for the knife, then abruptly stabs Philip repeatedly and slashes him across the mouth. In the apartment across the way, Grace Collier, a columnist for the small Staten Island Panorama newspaper, watches the attack in horror from her window and witnesses Philip crawl to a window to scrawl “help” in his own blood. Upon telephoning the police, Grace is annoyed when, recognizing her as the writer of a column accusing the police of brutality, they hesitate to believe her report. Reviving in the bathroom, Danielle answers a pounding on the door and finds a stunned Emil, who glances around at the bloody floor. When Danielle bewilderedly reveals that Dominique is visiting, Emil hastily orders her to help him clean up. Discovering Philip dead by the window, Emil and Danielle place the body inside the pullout sofa. Emil then hurriedly cleans all the blood away and carefully places the stained knife in a plastic bag. Meanwhile across the street, police detectives Kelly and Spinetti meet the anxious Grace, who insists on accompanying them to Danielle’s apartment. As Emil slips away unseen with all the bloody evidence, the detectives question Danielle, who reluctantly allows them into the apartment. Frustrated by Kelly’s refusal to allow her to interrogate Danielle, Grace explores on her own and begins to suspect that there may have been another woman in the apartment. Discovering the birthday cake in the refrigerator with two names written on it, Grace eagerly brings it to Kelly only to trip, dropping and ruining the cake. After apologizing to Danielle for disturbing her, Kelly threatens to charge Grace with making false accusations, and departs. Exasperated by her inability to prove the murder, Grace is roused when, driving by a bakery, she recognizes its name from the birthday cake box. Presenting her credentials from the Panorama , Grace questions the shop attendants, Elaine and Louise, and confirms that Philip had the names “Dominique” and “Danielle” written on the cake. Later, when her attempt to pitch the story of the racist police force disregarding the murder of a black man by a white woman fails to get attention from her editor, Grace hires private detective Joseph Larch to validate her murder claim. Disguised as a window cleaner, Larch gets into Danielle’s empty apartment and smuggles out a briefcase he finds there just as Emil and Danielle return. As he departs he overhears Emil ordering a moving company to ship the pullout sofa to Canada. Presenting Grace with the briefcase, Larch then relates his conviction that the dead body must be in the sofa, because when he tried to shift it to look for clues he found it unnaturally heavy. Examining the briefcase, Grace discovers a medical file on Danielle and Dominique Blanchion. After Larch departs to follow the moving van to discover who will retrieve the sofa in Canada, Grace meets with writer Arthur McLennon, who wrote a Life magazine story the year before on the Blanchion twins. Enthused that Grace is doing a follow-up to his own piece, McLennon shows her film footage of a never-broadcast documentary on the Blanchions, Canada’s first Siamese twins. The film reveals that Danielle and Dominique were born with the base of their spinal cords fused together and were raised in the church-run Loisel Institute after their parents died in an auto accident a year after their birth. Although the sisters were physically healthy, as they reached adulthood, they gradually developed psychological problems over their unique situation. The film ends with the news that the Blanchions were to undergo a separation operation. Grace excitedly notes that Emil is one of the physicians in the footage and asks why McLennon did not do the natural follow-up article. McLennon admits that after the Blanchions’ surgery he was abruptly barred from further contact with the twins or anyone from the institute, but upon bribing a surgical nurse he learned that Dominique had died on the operating table. At home that evening, when Grace hears nothing from Larch, she resumes spying on Danielle’s apartment and later follows Danielle and Emil to an isolated clinic in the country. After watching Emil subdue the agitated Danielle with an injection, Grace is shocked to see him pull a bloodied knife wrapped in plastic from his coat. Realizing the knife is evidence of the murder, Grace slips into the building to telephone the authorities but a paranoid patient who refuses to let her use the phone confronts her. A young attendant, Yansen, intervenes but ignores Grace’s overwrought demand to contact the police. When Yansen summons Emil, he lies that Grace is a new patient admitted the previous evening. Protesting furiously, Grace is led away and given an injection. Later, with Grace in a semi-conscious state, Emil repeatedly tells her she was mistaken about witnessing a murder and since there is no body, there has been no murder. Now aware that Danielle is on the bed beside her, Grace listens as Emil explains that as a physician treating the Blanchions at the Loisel Institute, he fell in love with Danielle who grew increasingly resentful of Dominique’s constant presence. To appease Danielle, Emil gave Dominique drugs to put her to sleep while he was with Danielle. Partially reviving during Emil’s chronicle, Danielle then explains that upon learning that she was pregnant, Dominique responded violently and attacked Danielle with gardening shears, resulting in the emergency surgery, which was necessary to save their lives. In a drugged stupor during Emil and Danielle’s account, Grace hallucinates that she is Dominique and is attached to Danielle. Dreaming that Emil is about to conduct the separation operation with a huge cleaver, Grace awakens screaming in terror, forcing Emil to give her another injection. Emil then tearfully embraces Danielle, declaring that her refusal to accept Dominique’s death during the surgery has driven her into a psychosis, which causes her to assume Dominique’s personality and become violent when confronted with sexual intimacy. Reacting when Emil then kisses her, Danielle breaks away from him but when Emil demands that she admit Dominique is dead and that she murdered Philip, Danielle attacks him with a scalpel. Traumatized by watching helplessly as Emil bleeds to death, Grace becomes comatose. Some time later after Grace has partially recovered and is resting in her mother’s home, Kelly visits her to report that they have arrested Danielle for Emil’s murder, but need details of the other killing. Although Kelly apologizes for not believing her earlier report, Grace adamantly insists that since there is no body, there was never any murder, unaware that she is parroting Emil’s instructions. Despite Kelly’s coaxing, Grace staunchly refuses to admit witnessing the earlier murder. Meanwhile in Canada, Larch has tracked the sofa’s delivery to a small, desolate train station and remains to watch it in anticipation of solving the crime.