Advertising salesman Philip Woode is the victim of a prank on a Candid Camera-style television show and wins dinner for two at a Manhattan restaurant. When Danielle Breton, a young French Canadian model and aspiring actress who was part of the prank, approaches him and suggests he take her along, he agrees. At dinner, they are interrupted by Danielle's ex-husband, Emil, who also follows them to Danielle's Staten Island apartment. After tricking Emil into leaving, Philip and Danielle make love on the sofa, during which a huge scar on her side is revealed.
After they spend the night together on the fold-out sofa bed, Danielle wakes up in pain and goes to the bathroom, where she takes two pills, leaving two more on the sink. She hears her sister Dominique call from the bedroom in French, and they argue offscreen, mentioning that after they were "separated," Dominique was put in a hospital "full of lunatics." The argument wakes Philip, who dresses in the bathroom, where he unwittingly knocks the pills down the drain. Danielle explains to Philip that Dominique, her twin sister, has come to visit to celebrate their birthday. At her request, he goes to the drug store to refill her prescription, also picking up a birthday cake on which he has "Happy Birthday Dominique and Danielle" written. Danielle, in agony, calls Emil for help. When Philip returns, Emil is watching from his car. Seeing who he believes to be Danielle asleep on the sofa bed, Philip brings the cake and a large knife to her, but is repeatedly stabbed by the woman - the crazed Dominique. He drags himself to a window, where he tries to alert a neighbor whose window faces Danielle's by writing "help" in his own blood, but eventually dies.
The neighbor, a reporter named Grace Collier, sees Philip and calls the police. In split screen, she is seen waiting for the police and trying to convince them of the murder she witnessed while Emil helps Danielle clean up and hide Philip's body by folding it inside the sofa. Grace accompanies the skeptical Detective Kelly and his partner on a search of Danielle's apartment, but Danielle insists that she has been alone since last night, and they find no evidence. Grace finds the cake in the refrigerator, but trips and drops it as she brings it to show the detectives, destroying the message. Undeterred, she goes to the bakery and talks to Louise Wilanski (Olympia Dukakis) and Elaine D'Anna (Justine Johnson), who remember Philip and the cake and the message in the decoration.
Certain that Danielle is hiding the murderer, Grace persuades her editor to let her investigate the story on the basis that the police are ignoring her because Philip was black. Her editor convinces her to work with Larch, a private investigator. He gains access to the apartment and figures out from how unusually heavy it is that the couch contains the body. He also finds a thick file from the Loisel Institute on the Blanchion Twins, Canada's first conjoined twins, which leads Grace to Life magazine reporter Arthur McLennen. He tells her that the twins were separated only recently, and that Dominique apparently died during the operation.
As Larch pursues the truck that Emil called to haul the couch away, Grace tails Emil and Danielle to a mental hospital. When she is caught, Emil convinces the staff that she is a new patient. He sedates and hypnotizes her, conditioning her to say "There was no body, because there was no murder." He promises to reveal everything, placing Danielle on the bed beside her, and Grace has a bizarre dream about the twins' past and their separation, in which she herself is Dominique. Emil tells Danielle that the separation was necessary because Dominique stabbed the pregnant Danielle with garden shears. Emil claims he then had to separate the twins to save Danielle, but Dominique died during the surgery.
Grace wakes up screaming, but Emil coaxes her back to sleep. As Danielle calls for her sister, Emil tells her that she now dissociates to a violent "Dominique" personality whenever she makes love to anyone. Emil kisses her passionately, bringing "Dominique" out so he can question her about the murder. She slashes him in the groin with a scalpel, and he bleeds to death, controlling her. Their bodies pin Grace to the bed. Grace awakens to find the sorrowful Danielle tenderly embracing Emil's bloody body and screams in horror. Detective Kelly arrests Danielle, who denies knowledge of the murders and says that her sister is dead.
Kelly interviews Grace, who is still under Emil's hypnotic spell, repeating the lines he fed her and denying there was a murder. However, Larch has tracked the sofa to a remote train station in Canada.
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