Jacki and Julian Dewar, a pair of fraternal twins, arrive by bus in London during their spring break from university, while their father remains in Mexico on business. They dive into the city’s underground party scene, bouncing between clubs and strip bars, always accompanied by Jacki’s teddy bear Agamemnon, a strange sort of father figure they both openly regard. At one club they meet Clive, a small-time pimp who survives by ingratiating himself with the wealthy and well-connected. Clive quickly ingratiates himself to Jacki, while Clive’s sometimes girlfriend Denise tries to seduce Julian. He rebuffs her, intent on pursuing an incestuous closeness with Jacki, convinced that he and his sister form two halves of a single hive mind and that their bond justifies a taboo expression of their connection.
It becomes clear that Clive is using the twins’ house as a hideout from a gangster to whom he owes a large gambling debt. One night, after Jacki rejects Julian’s advances, Clive drugs them with whiskey and marijuana and takes Julian to a brothel that houses his personal “Circus”—a secretive troupe of transvestite prostitutes who work for him. On Clive’s orders, two of the prostitutes forcibly assault Julian, and Clive photographs the acts. Clive then tries to blackmail Julian with the incriminating images to pay off his debts. Meanwhile, Denise tells Jacki that Clive has a history of raping and blackmailing men, and in some cases selling them into sexual slavery when they could not meet his demands. That night, Jacki consoles Julian, assuring him that their connection remains intact despite what has happened.
The following night, the twins convince a drunken Clive that he cannot tell them apart. He agrees, and they transform the room into a ritual space—setting up an altar for Agamemnon and wrapping themselves in modified bed sheets to resemble ceremonial robes. When Clive falters in telling them apart, the twins fatally stab him with Jacki’s antique Tantōs. In the ensuing violence, Agamemnon is cut in half, and the sight drives Jacki to a breakdown, forcing her to flee while Julian remains behind.
Jacki is found semi-catatonic on a dock by James Harrington-Smith, a member of parliament who recognizes her from a party. Amnesia takes hold as she recuperates at James’s flat, trying to reconstruct the night of Clive’s death. Her discovery of Clive’s body during a return home triggers a citywide manhunt, and James lies to the police about Jacki’s whereabouts that night, effectively shifting the blame entirely onto Julian.
As the plot unfolds, Jacki deduces that Julian has sought refuge in the same hotel where Clive kept his Circus. She promises James that she will persuade him to surrender and will contact the police if she does not return within an hour. The confrontation between the twins grows increasingly strained as Julian’s mind frays, convinced that their problems stem from a denial of their incestuous bond, which he believes would sanctify their uniqueness to the world. James, wary of a political scandal tied to the twins, opts to stay silent rather than alert the authorities.
Jacki nevertheless attempts to guide Julian toward escape from London, planning to reunite with James once she has located Julian and learned his whereabouts. The tension peaks as Julian, consumed by paranoia and delusion, attacks Jacki and chokes the life out of her. In a final moment of clarity, a grieving Julian mutters an apology to the dead Jacki, then extinguishes the room’s pilot light and ends his own life by gassing himself to death.
