An attractive young women walks into Poirot’s office seeking to hire him for a case. How many mystery or crime novels begin that way? This one has some unexpected twists. Carla Lemarchant believes her mother innocent of murdering her father. The murder took place sixteen years earlier, when Carla was a young girl. Her mother was convicted of poisoning her father with a hemlock derivative, coniine, which her mother had stolen from the home laboratory of a family friend, ostensibly to kill herself because her husband, famous artist Amyas Crale, was having an affair with a young model, one in a series and was planning to divorce her. The mother, Caroline Crale, died in prison shortly after, but before dying had written a letter, to be given her daughter when she reached adulthood, stating her innocence.
What makes this more compelling for Poirot is that Carla is engaged but the couple is apprehensive of having a husband poisoner in their background. So Poirot takes the case. His initial investigations turn up two basic facts, the case was open and shut, the poison being in a glass brought by the wife with a cold beer, and the lack of any real defense on her part during the trial apart from the claim that her husband must have committed suicide.
The plot then moves forward with interviews with the five people present at the time of the murder–the five little pigs! This is followed by five written accounts from each of these people, then the denouement, in the room where the poison was compounded. In all this, Poirot is not only looking for the discrepancies in accounts but also to understand the character of Caroline, as well as the others present. In this, he believes, the real answer lies.
From all accounts, it looks as if the wife is guilty, but Poirot thinks otherwise and the fun is seeing how he reaches that conclusion and exposes the real killer!