London, 1985. Richard Gillespie arrives late for a meal with his elder sister Natalie. The siblings were brought up separately and only occasionally see each other now their parents are gone. During the night, Richard wakes to discover Natalie crying. She is upset after recently finishing a relationship. They hug and she asks him to kiss her.

Two years later. Natalie is frustrated working as a secretary, while Richard enjoys a hedonistic lifestyle as an urban planner in Scotland. Natalie marries a wealthy entrepreneur, Sinclair, and moves into a large house in the Home Counties. Richard gives up his well-paid job to work for an agency, monitoring the new developments around London's Docklands.

Invited to lunch at their riverside house, Richard notices that Natalie has become more attractive and sophisticated in her new surroundings. The physical attraction between the two siblings grows. Natalie visits Richard at his Docklands flat. Before long, their relationship develops into a sexual one.

Richard meets Sinclair for lunch. Back at his work, Richard discovers his boss, Colin, is seriously ill with AIDS. Richard is invited to a family picnic in the countryside, where he attempts to talk to Natalie alone. They arrange to meet secretly in London. Natalie tells Sinclair that she will be in Nuneaton for the weekend on business.

At the rendezvous, Richard and Natalie reignite their sexual passion. Richard is concerned that their actions are illegal, while Natalie appears unperturbed. Sinclair becomes suspicious when he discovers Natalie is not registered at the hotel where she is supposedly staying.

Wandering through the grounds of a church, Natalie realises Richard is becoming obsessed with their relationship. She suggests that he finds himself a girlfriend. Natalie returns to her country house and Sinclair. Richard strikes up an acquaintance with a young woman, Paula. He phones Natalie to inform her he has found a girlfriend.

Richard and Natalie meet at a hotel. She tells him that Sinclair is suspicious and they should not see each other for the immediate future. Richard visits Colin in hospital and takes him out for the afternoon to a business meeting.

Richard discovers Natalie is out when he visits the country house. Greeted instead by Sinclair, he is invited on a boat cruise of the river. Sinclair tells him he knows Natalie is having an affair. Richard feigns ignorance. Upon returning to the house, Sinclair tells Richard that he and Natalie are moving to America shortly. Richard is devastated.

Richard makes a scene at a restaurant, pleading with Natalie to stay. Back at his flat, he swallows a large quantity of pills in desperation. Natalie arrives and tries to comfort him.

At Sinclair and Natalie's farewell party, Richard is moody and resentful. He physically attacks Natalie, accusing her of using him. Natalie reveals that they are not moving to America. Returning to the party, Sinclair reveals his knowledge of their affair. As the three of them walk by the river, Richard seems to accept his obsessive love for Natalie is now over.