Barbara, a veteran history teacher at a comprehensive school in London, is a lonely, unmarried woman in her early sixties, and she is eager to find a close friend. However, she reveals that she has been unable to make a previous friendship last as she was considered by previous friends to be domineering and demanding. Her former friend, teacher Jennifer Dodd, even threatened her with an injunction if she tried contacting her again. When Bathsheba “Sheba” Hart is hired as an art teacher, Barbara immediately feels that they might become close friends. When Sheba invites Barbara for Sunday lunch with her family, she is ecstatic and gives the lunch date enormous significance.
Initially unknown to Barbara, Sheba falls in love with a 15-year-old pupil, Steven Connolly, who is from a deprived background and has literacy problems. Although they frequently have sex in risky places, including at school and in the open on Hampstead Heath, the unlikely couple successfully conceal their affair from colleagues and family. During Barbara’s first visit to Sheba’s residence, she tells Barbara a highly expurgated version of what has happened between her and Connolly, claiming only that he has tried to kiss her and that she discouraged his advances. Barbara offers her some advice on how to cool the boy’s ardour, and considers the matter closed.
Sheba confesses to Barbara that despite her apparently charmed life, she feels unfulfilled. Sheba has a difficult relationship with her rebellious seventeen-year-old daughter, Polly, whose youth and beauty only intensify her mother’s own feelings of aging and waste. Sheba’s husband, Richard, is significantly older than she is, and their relationship sometimes has a father-daughter element to it. Sheba’s complaints trouble Barbara, who had considered Sheba to have the perfect family life.
Barbara eventually finds out about the affair on Guy Fawkes Night, when she sees Sheba talking to Connolly on Primrose Hill. Barbara feels betrayed that Sheba did not confide in her properly about this during the early stages of their friendship, and is angered by Sheba’s obsession with Connolly and her relative neglect of their friendship. When Barbara’s cat Portia is diagnosed with colon cancer and needs to be euthanised, she seeks out Sheba for support, only for her to get a call from Connolly during their meeting. Sheba abandons a distraught Barbara in favour of Connolly, compounding Barbara’s sense of betrayal from her.
The power dynamics in the relationship between Connolly and Sheba are changing, with Connolly’s interest in the affair waning as Sheba’s grows. Sheba becomes needier and starts to write love letters to the boy. Sheba secretly visits Connolly in his parents’ council house, and Connolly callously insults her when she tries to make conversation with him. Sheba still does not break off the affair, having become quite enslaved to the now barely invested Connolly.
Brian Bangs, a mathematics teacher, asks Barbara to have Saturday lunch with him one day just before Christmas. He confesses his infatuation with Sheba, leading Barbara to realise that he only asked her out to use her as a means to discover information about Sheba’s private life. Overcome by jealousy, Barbara alludes to Sheba’s secret. (“‘Sheba likes younger men, you know. Much younger men.’ I paused a moment. ‘I mean, you are aware of her unusually close relationship with one of the Year Eleven boys? ‘”) Afterwards, Barbara is wracked with guilt, but cannot summon up the courage to tell Sheba what she has done. Rather, she hopes Bangs will not report what she has told him.
Sheba’s relationship with Polly deteriorates further when the girl’s unruly behaviour results in her expulsion from her boarding school. On two occasions, Polly accuses Sheba of having an affair. Sheba is furious about the accusation, believing that she has covered her tracks successfully. Meanwhile, Sheba attempts to take comfort in Connolly throughout her troubles by calling him constantly, and when he fails to answer her calls, she shows up again at his house, only to find that Connolly has taken up with a younger girl and abandoned her. Even this new found evidence of Connolly’s loss of interest in her is not enough for Sheba to cease her obsession of him, and she pines for him even knowing she will never hear from him again.
The school’s headmaster is somehow informed about the illicit affair — it is implied that the culprit is Bangs. Sheba is suspended from her job and charged with indecent assault on a pupil. Her husband demands that she leave the family home and prevents her from seeing their children, especially their son Ben, who has Down syndrome; Polly, meanwhile, refuses to have any contact with her. While Sheba’s life is quickly disintegrating, Barbara thrives on the new situation, which she considers her chance to prove her qualities as a friend, even when the headmaster, glad to rid himself of one of his severest critics, forces her into early retirement. Barbara gives up the lease on her own small flat and moves with Sheba into temporary accommodation in Sheba’s brother’s house.
Sheba finds Barbara’s manuscript and discovers that she has been writing an account of her relationship with Connolly. She is distraught and furious, not least because Barbara has written about events she did not personally witness, and made judgements about people close to Sheba. She is eventually reconciled with Barbara due to their shared desperation and loneliness, as both have now lost everything. Even now, Barbara uses their desperate circumstances as yet another opportunity to further their relationship, and the mentally weakened Sheba can do little to resist. The novel ends with Sheba, trapped and demoralised, resigning to Barbara’s dominance of her.