Barbara Covett, played by Judi Dench, is a history teacher at a comprehensive school in London, leading a reclusive life dominated by her diary. Her world is turned upside down with the arrival of a vibrant new teacher, Sheba Hart, portrayed by Cate Blanchett. Drawn to Sheba, whom she feels could perhaps fill the void in her life, Barbara soon discovers that Sheba is involved in a scandalous affair with a student, Steven Connolly, played by Andrew Simpson.

When confronted by Barbara, Sheba pleads for discretion until Christmas to preserve her family’s peace. Barbara, however, has no intention of informing anyone, provided Sheba puts an end to the affair. As time passes, Barbara’s obsession deepens—she manipulates and emotionally blackmails Sheba, creating a rift in her family life and drawing Sheba further into her emotional turmoil. The situation intensifies after the death of Barbara’s cat and Richard, Sheba’s husband, expresses his frustrations, pinpointing Barbara as a source of distress in their marriage.

In a moment of vindictiveness, Barbara, feeling rebuffed and alone, reveals Sheba’s indiscretions to a fellow teacher. This betrayal spirals out of control, leading to a confrontation with Mrs. Connolly, Steven’s mother, who assaults Sheba. This culminates in a profound argument between Sheba and Richard, which Barbara relishes in, knowing it will drive Sheba back to her. Soon, chaos ensues as Sheba’s life unravels—her marriage collapses, relationships with her children are severed, and she faces the repercussions of her actions at work.

After being dismissed from her job due to her deceitful manipulations, Sheba mistakenly believes that the affair was exposed by Steven’s confession, only to later discover Barbara’s treachery through her diary. This revelation leads to a volatile confrontation where Barbara claims to have liberated Sheba from her dissatisfaction with her marriage. Their relationship implodes amidst accusations of manipulation and betrayal.

In a defining moment, Sheba, struggling with her emotions, rushes out into a mob of reporters, prompting Barbara to come to her aid. Despite their turbulent connection, Sheba confesses that their friendship was genuine—something Barbara seems unable to reciprocate. As Sheba returns to her family, Barbara’s duplicity is front and center, and she casually entertains a new acquaintance, Annabel, as their story leaves traces of her past with Sheba—the thrill of obsession, heartbreak, and dark manipulation ever lingering in the background.

The final scenes illustrate Barbara’s chilling need for companionship, alluding to the cycle continuing as she invites yet another woman into her life, effectively downplaying the truth of her prior connection with Sheba.