In 1926, Agatha Christie finds herself in a difficult place when her writing is thwarted by predictable plotlines 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.