A young woman named Carla Lemarchant needs Hercule Poirot's help. Sixteen years earlier, her father died from poison. Her mother was convicted of the crime and died after a year in prison. Carla fears that the man she is about to marry will always view her with suspicion unless her mother's name can be cleared. Poirot must delve deep into the past to find the real truth. Was it really murder? Suicide? Or, for sixteen years, has someone lived with the knowledge that he — or she — has committed the perfect crime?