As Hercule Poirot stands over the body of Lavinia Seagram on her blood-soaked bed, it seems the former Nanny has succumbed to her alcoholism and committed suicide. But a discovery connects her to Norma Restarick, the troubled young heiress who that very morning had confessed to Poirot that she might have killed someone... Norma is the Third Girl sharing a flat with her father's secretary Claudia and the bohemian model Frances, two beautiful young women whose world comprises parties and bright futures - a world Norma struggles to fit in to. In this gripping tale of passion, mistrust and obsession Poirot soon finds that everyone close to Norma holds fears for her sanity, and both the elderly Sir Roderick and her father Andrew believe the answer is to be found in a haunting tragedy from her past. As Poirot matches his wits against the case, the threat of death comes closer to home when his friend Ariadne Oliver trails the arrogant artist David Baker through the maze of London's streets and finds herself the victim of attack. And when a surprise announcement is to be made at the Restarick's country mansion, it is certain that the murderer will make their final move. Taking life and death in his hands, Poirot lays a trap that will finally solve the mystery of the Third Girl, but that may do so at the greatest cost imaginable...