Identical twins Elliot and Beverly Mantle share a deep, symbiotic bond. They affectionately call each other “Elly” and “Bev,” as Elliot, the more socially confident brother, takes the lead. Growing up in Toronto, Canada, the boys are fascinated by female reproductive organs. One day, they ask a little girl to have sex with them in the bathtub “as an experiment,” but she refuses. Later, using mini surgical instruments, they play doctor, working side by side on an anatomical model of a woman. In 1967 Cambridge, Massachusetts, the twins invent an instrument called the “Mantle retractor” as undergraduates and, although their professor initially dismisses their concept, the instrument wins a prize and goes on to become an industry standard. In 1988 Toronto, Beverly and Elliot have established their own fertility clinic and live together in an apartment. When famous actress Claire Niveau visits the clinic, Beverly finds that she has a rare condition, in which her womb is trifurcated, with three entrances to the uterus instead of one. Beverly urges Elliot to examine Claire, but he only agrees once he recognizes the actress’s name. As Elliot impersonates Beverly in the examination room, Beverly attends a fundraising dinner in his brother's place. That night, Claire has sex with Elliot, believing he is Beverly. When Elliot returns home, he tells Beverly, the more sensitive and intellectual twin, to take his place at a lunch date with Claire the next day, promising that she is a great lover. At Claire’s apartment, Beverly breaks the news that she will not be able to conceive. She makes sexual overtures to Beverly, unaware that he is not the man she made love to the night before. Alarmed by Claire’s masochistic sexual requests, Beverly is reluctant to continue seeing her, but Elliot persuades him to do so. One day, Beverly binds Claire with medical tubing and surgical clamps, and, after their passionate lovemaking session, he promises to keep their sex life a secret. Returning home, Beverly refuses to tell Elliot what happened, but Elliot argues that Beverly’s experiences are not complete until he shares them with his twin. Posing as Beverly again, Elliot goes to Claire’s apartment, where she admits to taking drugs. Elliot agrees to procure them for her but later tells Claire that she is hustling him for drugs. Although Elliot suggests that Beverly should stop seeing her, Beverly refuses. One day, Beverly helps Claire learn her lines, and she remarks that he is a self-conscious reader. She asks if he has ever done any impersonations, and he accidentally replies that he has only ever impersonated his brother. Surprised to learn he has a sibling, Claire prods for more information, but Beverly loses his temper. Bewildered, Claire says she sometimes likes Beverly very much, but other times he is no more than an amusing lover. Beverly takes one of Claire’s pills to calm his nerves. Soon after, a friend inadvertently reveals to Claire that Beverly has a twin brother. Claire confronts Elliot, believing he is Beverly, and asks to see the twins together. Sitting down with the two of them at a restaurant, Claire accuses them of deceiving her and throws water at Beverly. After she storms out, Elliot chuckles while Beverly weeps. Heartbroken, Beverly makes a drunken scene at an award ceremony honoring his brother. Elliot accepts an associate professorship, forcing Beverly to take on more work at the clinic. After a chance encounter, Beverly apologizes to Claire and they reunite. Sleeping in her bed, Beverly has a nightmare that he and Elliot are Siamese twins conjoined at the stomach, and Claire is gnawing at the flesh between them. One day, Claire invites Elliot to her movie set to tell him about Beverly’s growing drug addiction. Announcing that she must go away on a movie shoot, she asks him to look after his twin. Elliot suggests that Claire presents a distracting element to the “Mantle brothers’ saga,” and proposes that he and his brother share her. Claire rejects the offer, insisting that Beverly is very different from Elliot. While Claire is away, Beverly phones her hotel and assumes she is having an affair when her male secretary answers. Elliot comforts Beverly and insists that his relationship with Claire cannot be love if it makes him feel so bad. At home, Elliot tries to interest Beverly in a ménage à trois with his casual girl friend, Cary, but Beverly rejects the idea before collapsing onto the ground. After a trip to the hospital, Elliot supervises Beverly’s detoxification by himself, hoping to avoid a scandal. Beverly secretly continues his drug use and becomes insensitive, delusional, and sadistic toward his patients. While giving a woman an internal examination, he uses the Mantle retractor, which is only intended for surgical use. He later commissions prototypes of disturbing-looking instruments from a metal artist. Dressed in a scarlet surgical gown, Beverly unveils the new instruments during a surgery, but stops short to inhale his patient’s anesthetic, prompting the attending doctors to tear him away from the operating table. At the hospital inquiry, Elliot impersonates Beverly, claiming to be overworked. Elliot learns that Beverly’s instruments have been confiscated by hospital authorities, and later, asks his brother why he used them. Beverly claims that his patients’ insides are deformed and therefore require special instruments. Taking his brother under his care once again, Elliot asks Beverly to recall the story of the original Siamese twins, Chang and Eng. Beverly remembers that Chang was the sickly twin who drank too much, and when Eng realized that Chang had died in bed, Eng died of fright. Cary recommends that Elliot distance himself from Beverly, but Elliot says they must become synchronized. Claire calls Beverly when she is back in town, and convinces him that she was not having an affair. On his way to her apartment, Beverly sees copies of his gynecological instruments in an art gallery window and steals them from the display. With Claire, Beverly returns to partial sobriety. He worries that he has not heard from Elliot in almost a week. She asks about the surgical instruments, and Beverly says they are for separating Siamese twins. Beverly goes to the clinic where Elliot has descended into a drug-fueled chaos, and the roles are reversed as Beverly cares for Elliot. They plan a last weekend of excess and celebrate with cake and soda. Later, as Elliot sits on a gynecological procedure chair, Beverly carries in his instruments and announces that he will perform a separation surgery, addressing his brother as “Chang.” Elliot requests drugs, and Beverly gives him an injection before making an incision into his abdomen. Beverly cries and says that separation can be a terrifying thing, but Elliot assures him they will always be together. As Elliot lapses into unconsciousness, Beverly continues with the operation. He later awakes near Elliot’s dead body and calls out to his brother, saying he had a terrible dream. Beverly leaves the clinic to call Claire from a payphone; however, he hangs up after she answers and walks back inside. Sometime later, Beverly lies lifeless on the floor, his dead body curled over Elliot’s.