A publishing house in London. The atmosphere is strained between manager Peter and his secretary Sarah, who have just started having an affair. He can't get away very often to see her. Sarah has lunch with Geoff, with whom she has a casual relationship. He tells her he's leaving the country to work in Frankfurt.

Peter's daughter, Prue, returns from her summer holidays and calls in at the publisher's in order to surprise her father. But Sarah greets her frostily and deliberately forewarns Peter of Prue's arrival. Peter has lunch with Prue, but seems distracted, and later complains to Sarah that Prue always gets her own way.

Back at her flat, Prue complains to Gavin about her father's sudden coolness towards her. But Gavin reveals that Peter and Sarah were caught in bed together in this very flat. Prue is deeply upset and determines to inform her mother, Cassie. But Gavin thinks this will hurt Cassie unnecessarily and threatens to hit Prue if she carries out her plan.

Peter installs Sarah in a plush new apartment so they can meet more easily. His name is on the lease and he offers to pay her rent. She sees Geoff to the airport, then moves out of her old flat into her new home. On the first night, she receives flowers from Peter but feels lonely.

The next day is Prue's 20th birthday. She's to celebrate it at the family home, where Cassie is preparing a meal. Prue and Gavin argue on the train, and Gavin phones Cassie in advance to warn her of Prue's erratic behaviour.

Sarah is visited by her sister, Barbara, who is impressed by Sarah's status as a kept woman and bemoans the lack of excitement in her own life.

Meanwhile, Prue is presented with her birthday present, a gold bracelet picked out for Peter by Sarah. The family sit down to dinner and Prue starts making cryptic allusions to Peter's antics, building to a blunt revelation of his infidelity.

Later, Gavin makes good on his promise and beats Prue savagely in the woods.