Agatha and the Truth of Murder (2018)

In 1926, Agatha Christie finds herself in a difficult place when her writing is thwarted by predictable plot lines and her unfaithful husband pushes her for a divorce she does not want.

As she searches for an alternative creative route to revive her novel development, she is approached by a woman, Mabel Rogers, seeking help in solving the murder of her partner, Florence Nightingale Shore, who had been bludgeoned on a train. Though initially reluctant to facilitate a private investigation, Christie goes undercover while the nation searches for her whereabouts. She soon finds herself taken to a new story with real characters and tangible danger. Calling herself "Mary Westmacott" (an early Christie pseudonym), she gathers all the suspects in the attack in a country house under the pretext of determining their share of a large inheritance from a fictitious American businessman, with herself as the representative of a law firm and Mabel acting as housekeeper and cook. The suspects include Daphne Miller, a young woman whose nursing career Florence could have ruined if she had lived, Randolph, Florence's cousin who inherited her money, Zaki Hanachi, a French soldier of Algerian ancestry who she helped recover after the war, and who may have been asking her for money, Travis Pickford, a boxer and black marketer who was interviewed by the police, and Mrs. Pamela Rose, the woman Florence was travelling to see. Daphne is accompanied by her abusive father Wade, and Mrs. Rose by her son Franklin, a former chaplain. Mabel is able to search the guests' bags, and finds that Wade Miller may have a pistol with him. To try and force a reaction from the others, "Mary" announces after the first interview that Daphne will get the biggest share of the inheritance. Events take a harrowing turn when Daphne's father is shot dead. Detective Inspector Dicks arrives with a single constable, complaining that he is shorthanded because of the hunt for Agatha Christie, and that because of this he must conduct the investigation at the house instead of the police station. It is quickly discovered that they have been gathered together under false pretenses, and "Mary" changes her story to suit. Mabel is found in possession of the murder weapon and arrested. Dicks reveals to "Mary" that he knows that she is Agatha Christie. He also knows that Daphne shot her father, based on an injury to her hand from the recoil of the automatic pistol. Christie, having found a vital detail in Florence's diary, tells him she knows who attacked Florence, and asks for his help. They spring a trap to get the real killers to admit their involvement. Even after Dicks and others overhear the conversation, the evidence is too thin to convict them, despite there being proof that they wrote a note blackmailing Daphne into planting the gun in Mabel's room. Instead, the others conspire to frame Florence's killers for the murder of Daphne's father. Inspector Dicks helps Christie set up the cover story for her disappearance and inadvertently helps her get the idea for a new book. She is shown later completing a manuscript which is implied to be "Death on the Nile", though this was published some years later. (copied from Wikipedia)