A young woman stands in the dock, charged with poisoning the girl who took the man she loved. The case against her seems airtight: she alone had the motive, the opportunity, and access to the means. The story opens in the courtroom, then circles back to the events that brought her there. Cousins Elinor Carlisle and Roddy Welman are engaged and waiting to inherit from their wealthy, ailing aunt when an anonymous letter warns that someone is ingratiating herself with the old woman. Suspicion falls on Mary Gerrard, the lodge-keeper's beautiful daughter, whom the aunt has come to favour. When Roddy's feelings begin to shift toward Mary, the foundations of Elinor's life start to crack. After a death in the household and then a second, fatal poisoning, the evidence points squarely at Elinor. Only the local doctor, who loves her, refuses to believe in her guilt, and he turns to Hercule Poirot to uncover what the obvious reading of the facts has missed. Poirot arrives well into the affair, working through conversations, recollections, and small contradictions that the official account overlooks. Unfolding across the lead-up to the crime, the detective's inquiry, and the trial itself, the novel leans as much on jealousy, longing, and wounded pride as on method, building toward a verdict whose weight rests on a single overlooked detail.