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 a way to revive her novel development, she is approached by Mabel Rogers, who is seeking help in solving the murder in 1920 of her partner, Florence Nightingale Shore, who had been bludgeoned to death on a train.

Though initially reluctant to undertake a private investigation, Christie goes undercover while the nation searches for her whereabouts.

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.

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.

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.

[4] Locations included Grey Abbey House in Greyabbey,[5] Royal Belfast Golf Course, and Ulster Folk and Transport Museum.