In 1926, Agatha Christie, Ruth Bradley, finds herself in a difficult place as her writing stalls under predictable plot lines and her unfaithful husband pushes for a divorce she does not want. Seeking a spark to revive her novel, she is approached by Mabel Rogers, Pippa Haywood, who asks for help solving the 1920 murder of her partner, Florence Nightingale Shore, Stacha Hicks, who was bludgeoned to death on a train. Though initially reluctant, Christie agrees to go undercover, adopting the name Mary Westmacott, a pseudonym she has used before, and heads into a tense, public-facing game of disguise while the nation keeps tabs on her movements.
Christie gathers all the suspects in a country house, presenting a scenario that each guest has a stake in a large inheritance from a fictitious American businessman, with Christie posing as a representative of a law firm and Mabel acting as housekeeper and cook. The lineup is loaded with potential motives: Daphne Miller, Bebe Cave, a young nurse whose career Florence could have ruined if she’d lived; Randolph, Tim McInnerny, Florence's cousin who stands to inherit; Zaki Hanachi, Luke Pierre, a French soldier of Algerian ancestry whom Florence helped after the war and who could have felt betrayed or owed money; Travis Pickford, Blake Harrison, a boxer with a murky past including black-market dealings; and Mrs. Pamela Rose, Samantha Spiro, the woman Florence was traveling to see. Daphne is accompanied by her abusive father Wade, Dean Andrews, and Pamela Rose is joined by her son Franklin, Joshua Silver, weaving a web of leverage, threat, and secrets around the group.
As tensions mount, Mabel is found in possession of the murder weapon and is arrested, while Detective Inspector Dicks, played by Ralph Ineson, arrives with a single constable to oversee the case at the house rather than at a police station, a sign of how high-profile Christie’s involvement has become. It soon emerges that the gathering was built on false pretenses, and Christie shifts the narrative to suit the evolving needs of the investigation. She uncovers a vital detail from Florence’s diary that points toward a killer, and she uses it to press Daphne’s collaborators for a confession. Yet even with this breakthrough, the evidence remains fragile, and the plan to tie Florence’s killers to the murder of Daphne’s father falters as others conspire to redirect blame away from themselves and onto Florence’s circle.
The investigation twists into a wider deception, with Inspector Dicks helping Christie stage a cover story for her own disappearance, a gambit that doubles as inspiration for a fresh manuscript. The experience convinces Christie to channel what she has learned into a new narrative arc, culminating in a manuscript that bears the unmistakable fingerprints of a future bestseller and a nod to her celebrated work, Death on the Nile.
