This plot unfolds chronologically, although significant segments are recounted through flashbacks. In 1958, [Joan Archer](annie-starke), portrayed by Annie Starke, encounters Joseph Castleman (played by Harry Lloyd), a dashing married professor at a women’s college. Despite being a capable writer herself, Joan finds herself captivated by Joseph's commanding presence and his advice that "a writer must write." She meets a published alumna, [Elaine Mozell](elizabeth-mcgovern), whose cynical take on women's opportunities in literature leaves Joan feeling disheartened.
Fast forward two years, and Joseph has lost his job due to his affair with Joan, while his marriage crumbles, and his initial foray into novel writing turns out to be lackluster. Joan, now working as a secretary at a publishing company, witnesses the pervasive sexism as male editors overlook female authors. When Joan critiques Joseph's manuscript, he threatens to end their relationship, deriding her as lacking the capacity to love "a hack." In a moment of compromise, Joan agrees to revise Joseph's novel, which, after being named The Walnut, is released to great success.
By 1968, Joseph and Joan reside in an expansive seaside home in Connecticut, with Joan diligently crafting her own novel under Joseph's name. Joseph, in turn, carries the household responsibilities and nurtures their first child, [David Castleman](max-irons). Their conversations reveal that Joan's novel mirrors their shared life, a notion which ultimately bores her. Over the ensuing four decades, Joseph reveals his narcissistic traits through numerous infidelities, broadcasting to others that Joan "does not write."
Jumping to 1992, a now-elderly Joseph (portrayed by Jonathan Pryce) has gained fame as a celebrated author. He receives the Nobel Prize in Literature, but [Joan Castleman](glenn-close) appears visibly unenthusiastic about the accolade. In this dynamic, David idolizes his father and seeks Joseph’s feedback on his own budding writing endeavors, blissfully unaware that Joan has penned all of Joseph’s works. The trio heads to Stockholm, where [Nathaniel Bone](christian-slater), a biographer with a knack for scandal, tries to intertwine himself into their lives. Joan's discontent escalates as Joseph is lauded, with his public efforts to express gratitude for her support only serving to deepen her resentment.
Nathaniel, catching on to Joan's emotional turmoil, coerces her into a conversation over drinks, wherein he reveals his belief that she is the ghostwriter behind Joseph’s novels. Joan, while not confirming his claims, does not dissuade his suspicions. Meanwhile, Joseph pursues a young photographer, though his cardiovascular condition prevents him from consummating this new affair. Following an intense spat where Joseph accuses Joan of neglecting him, she counters with outrage over his infidelity. Their conflict reaches a temporary halt when they learn about the birth of their grandson, courtesy of their daughter [Susannah Castleman](alix-wilton-regan).
On the evening of the Nobel Prize ceremony, David challenges his parents after learning from Nathaniel that Joan is the true writer in the family. Both Joseph and Joan vehemently deny this assertion. However, as the night progresses, Joan becomes increasingly distressed with the accolades directed at Joseph, leading her to flee. In pursuit, Joseph implores Joan to accept his Nobel Prize, to which she firmly declines. At their hotel, she confronts Joseph with intentions of divorce. An explosive argument erupts, culminating in Joseph’s heart attack. Lying on the bed, he pleads for Joan’s affection while she professes her love, only for him to reply, “You’re such a good liar,” before passing away moments later.
On their flight back to the United States, Nathaniel expresses his sympathies to Joan, who sternly warns him against publishing anything that could tarnish Joseph’s literary reputation, stating she will take legal action if necessary. David happens to overhear this exchange, and Joan resolves to disclose the truth to her children once they arrive home.
