When the goddaughter and namesake of Florence Nightingale is found on a train to Hastings with her skull caved in, the subsequent investigation and public outcry fail to catch the killer. Driven to distraction by the lack of progress and the failure of the police, Florence’s partner of 26 years, Mabel Rogers, begins her own investigation. But having found a number of suspects, Mabel cannot find proof which will expose the murderer. At her wit's end she makes an appeal to the creator of Poirot, whose latest novel "The Murder of Roger Ackroyd" is an international sensation.

Agatha receives Mabel’s approach soon after her husband tells her he’s in love with his secretary and wants a divorce. This blow, following months of grind finishing "Roger Ackroyd" and a gruelling publicity tour, plunges Agatha into depression. To make matters worse she's terrified her ability to confound her audience is coming to an end. Readers, wise to her tricks, are starting to guess the culprit: it’s always the least-likely suspect.

Moved by Mabel's plea for help and desperate for escape and inspiration, Agatha agrees to try and solve Florence's murder. Taking a leaf from her own book, she lures the suspects to a country retreat on the promise of an inheritance. Then, to mask her disappearance and throw suspicion on her husband, Agatha abandons her car and travels in disguise, meeting Mabel and the would-be-murderers at a neglected mansion.

Pursuing a plan based on everything she’s learnt through her work, Agatha discovers the mystery of Florence's death is intent on writing itself; the killer is far more cunning than any of her fictional creations. As she battles to uncover the truth, press speculation about her disappearance reaches fever-pitch: thousands of policemen are searching for her and her husband is being questioned. Can Agatha solve the murder and save her writing before her cover is blown and her world comes crashes down?