In short order Barbara falls in love with Sheba, and a bond fortuitously forms between them when Barbara breaks up a fight in Sheba's class that the novice teacher plainly can't handle (and doesn't realize is about her). Sheba is so grateful she invites Barbara over for Sunday lunch with her older husband Richard (Bill Nighy) and their children — a petulant teenaged daughter named Polly (Juno Temple) and a son with Down's syndrome named Ben (Max Lewis), whom Barbara snidely pigeonholes in her diary as "a pocket princess" and "a somewhat tiresome court jester." After lunch, Sheba takes Barbara back to her home studio and confides lavishly in this woman she barely knows. The problem isn't that Sheba is imparting too much information but that she's unsuspectingly exciting Barbara, who will later preserve a fallen hair from Sheba's head between pages in her notebook and then save a seat at the school Christmas pageant for her as if they were attending it on a date.

When Sheba doesn't appear at the pageant, Barbara goes looking and spies her putting her clothes on after dallying with Steven Connolly (Andrew Simpson), a 15-year-old student. (In the book, when Barbara tells of the school's history as a Victorian orphanage it's hard not to think of Heller as giving sly new meaning to Dickens's Dotheboys Hall.) Barbara is infuriated by what she thinks of as a "betrayal," though it's only a betrayal of her fantasies, which Sheba is unaware of. Barbara's denial is so impenetrable that she expresses her anger at this "rejection" as inflexible rectitude. As she tells her diary, however, this "disappointment" actually presents an opportunity to insinuate herself further into Sheba's life.

Both women are unself-aware in ways that only entangle them further. After Sheba initially opens up to her after the lunch, Barbara (who is from a resolutely middle-class family but nonetheless "the more educated woman" compared to "posh" Sheba) writes mordantly of the "immediate incautious intimacy" in "bourgeois bohemia." This also means, however, that Sheba will be only too ready to confide every gradation of her affair to Barbara, as if the truth — which Sheba reveals in the light of her relatively innocuous habit of quasi-therapeutic self-examination — couldn't possibly harm her. Barbara makes Sheba promise to stop seeing Steven, which Sheba does without realizing what Barbara will read into this act of submission. In Barbara's mind, acutely observant and yet as emotionally turbulent as Polly's, this is the beginning of a love affair. For a living mummy like Barbara the absence of consensual signs and words is without significance.

Barbara makes her boldest physical move at Sheba's house just when Sheba is expecting Steven, whom she couldn't resist seeing again. (Sheba should know better, but it's made clear that this fox-eyed scamp with burning cheeks seduces her rather than the reverse. Sheba isn't a predator, merely a fool, though far less deluded and remote in the movie than in the book.) This allows Barbara to increase her demands, imagining that she'll be able to break Sheba away from her family and even that it's for Sheba's greater happiness that she's doing so. When those demands become too great for Sheba, as they had for Jennifer, a spent Barbara, smeared with dirt from grubbing Portia's grave into which she also throws an expensive gift from Sheba, tattles to a fellow teacher with the lethal cunning of a Borgia courtier. The repercussions extend beyond Barbara's expectations but she doesn't care because the two unemployed ladies end up alone together in Barbara's basement flat.