| Director | Brian De Palma |
|---|---|
| Budget | $500,000 |
| Box office | $1 million |
Advertising executive Philip Woode wins dinner for two at a Manhattan restaurant on a Candid Camera-style television show. Danielle Breton, a young French Canadian model and aspiring actress who was part of the prank, flirts with Philip and he agrees to take her as his date. After dinner, they retire to her Staten Island apartment to have sex. The next morning, Danielle tells Philip that Dominique, her identical twin sister, has come to celebrate their birthday. At her request, he goes to the drug store to refill a prescription and picks up a birthday cake at a bakery on his way back. When he returns, he is stabbed to death by the crazed Dominique. Before he dies, he tries to alert a neighbor by writing "help" in his own blood on a window.
The neighbor, a reporter named Grace Collier, calls the police. Meanwhile, Danielle's ex-husband Emil helps her clean up and hide Philip's body by folding it inside a sofabed. 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 the previous night. 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. She hires Larch, a private investigator, to gain access to the apartment. He determines that the couch contains Philip's body. He also finds a thick file from the Loisel Institute on the Blanchion Twins, Canada's first conjoined twins. Grace's own investigations uncover 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 Grace and places Danielle on the bed beside her, who is revealed to be barren due to an incident that Danielle refuses to talk about. Emil promises to "tell her everything": revelations that play out in a monochrome sequence in which Grace (in her drugged state) hallucinates herself to be Dominique.
Danielle and Dominique were orphaned conjoined twins. As adults, the shy, timid Dominique quickly became a burden to the more outgoing Danielle, who was married to Emil. After several sexual encounters, where Dominique is drugged unconscious against her will by Emil at Danielle's request so that the couple are uninterrupted, Danielle becomes pregnant with his child. Dominique snaps after finding out that her sister is pregnant and stabs her in the stomach to kill her fetus. Emil arranges for a surgeon to separate Dominique from Danielle, the latter of whom is now badly bleeding from the wound that has also left her barren. However, Dominique dies during the procedure, much to Danielle's horror. This causes Danielle to snap and create a split personality representing her sister. Whenever Danielle has a sexual experience, the violent "Dominique" personality takes over. The pills she took kept Dominique at bay.
Emil kisses Danielle passionately to bring forth "Dominique" so he can question her, but she slashes him in the groin with a scalpel and he bleeds to death. 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 lines that he fed her to deny there was a murder. However, Larch tracks the sofa to a remote railway station in Canada.