As Hercule Poirot stands over the body of Lavinia Seagram, it seems the former nanny has succumbed to her alcoholism and committed suicide. Poirot discovers that she is connected to Norma Restarick, the troubled young heiress who that very morning confessed to Poirot that she might have killed someone. Perhaps the great Belgian detective has stumbled on his next case.
Norma is the third girl sharing an apartment with her father's secretary Claudia and the bohemian model Frances, two beautiful young women who inhabit a world Norma doesn't quite fit into. Indeed, Norma seems haunted by dark memories of her childhood, and everyone around her fears for her sanity. But is the greater risk her physical safety?
Poirot, aided by his fiction-writer friend Ariadne Oliver, matches wits against a layered and compelling case, and through it is faced with a family's tragic past. But the menacing reality of the present looms — a murderer is at large, and Poirot may have to take life and death in his own hands to set a trap that will finally solve the case of the mysterious third girl.