In Littlehampton during the height of the suffragette movement in 1920, an atmosphere of unease unfurls as Edith Swan, a devout Christian spinster portrayed by Olivia Colman, faces a deluge of hate mail that undermines her sense of safety and dignity. This onslaught of vitriol causes significant distress not only for Edith but also for her oppressive father, Timothy Spall, and her gentle mother, Gemma Jones. The count of these letters is staggering, with a total of 19 brimming with crude language, compelling Edward to turn to the local authorities for assistance. They promptly zero in on Rose Gooding, an Irish migrant and single mother played by Jessie Buckley, whom they suspect to be the instigator of this malicious campaign, largely due to her reputation for colorful language.
The former connection between Edith and Rose, intertwined with a bond formed through their shared experiences, has been irrevocably strained. This shift stems from a raucous incident at a birthday celebration for Edward, where Rose's winning verbal exchange with a guest leads to the unraveling of their friendship, worsened by accusations that Rose had tipped off child protective services regarding Edith. As tensions escalate, Rose is unjustly arrested and can neither post bail nor ensure her daughter Nancy, played by Alisha Weir, is cared for, leaving her in the hands of Bill, her partner.
Despite her underlying skepticism about Rose's involvement, police officer Gladys Moss, interpreted by Anjana Vasan, finds herself impeded by her superior, Paul Chahidi, who dismisses her doubts based on misogynistic prejudice. Desperate for understanding, Rose reaches out to Edith’s friends Ann, Mabel, and Kate to gather insights into the situation. Kate's hostility contrasts with Ann and Mabel’s compassion, leading to Rose feeling isolated. While initially aiming to gain Gladys' support, Rose faces rejection, but eventually, it's her friends who come to her aid, bailing her out upon her release.
The unfolding drama rapidly catches the attention of the larger community, transforming the case into a national sensation that even reaches Westminster. It is at this critical juncture that Gladys resolves to take charge of her investigation, recognizing the systemic discrimination shared by both herself and Rose. Meanwhile, the spark of nefarious intent in Edith begins to shimmer through as she grapples with her smoldering resentment against her father, crafting a letter that inadvertently becomes a key clue in the case when intercepted by an unsuspecting Victoria, resulting in catastrophic consequences.
As police formalities conclude, Gladys begins to piece together striking resemblances between Edith's handwriting and the loathsome letters, yet her findings are brushed aside by Spedding. In defiance, she enlists Ann, Mabel, and Kate in a rogue investigation, deliberately excluding Rose. As the trial date looms, Edith hatches clever strategies to elude Gladys’s notice while posting another incriminating letter that could alter the outcome.
During the climactic trial, Rose confronts her past, taking ownership of her vocal outbursts that perhaps express her frustrations more suitably than the anonymous letters. Nevertheless, Rose's truth unravels and reveals that Nancy's lineage is not what she'd previously disclosed, sending shockwaves through her family. At the same time, Edith uncovers that her father had been the one to alert child services about Rose, casting an even darker shadow over their already strained relationship.
In a thrilling turn of events, Gladys and her confidants devise a clever plan involving stamps with invisible ink to capture Edith’s inevitable attempt to frame Rose once more. As tensions mount and Rose faces an unexpected arrest attempt, she manages to confront Edith, igniting a vehement confrontation that showcases the bitter resentment underlying their intertwined fates.
In an unexpected twist, the letter Edith dispatches for her own vindication falls into the hands of Gladys's niece, revealing the hidden ink and clinching Edith's downfall while liberating Rose from her wrongful blame. The aftermath witnesses a moment of reluctant reconciliation between Edith and Rose, where regret is acknowledged, yet so is the resolve against Edward's harshness.
Ultimately, as Edith is led away to serve twelve months of hard labor, Rose walks free and is spared further tumult, while Gladys gains well-deserved recognition for her relentless pursuit of justice, wrapping up a tale steeped in societal struggle and transformation.
