A young woman named Carla Lemarchant approaches Poirot with a desperate request. Her mother, Caroline Crale, was convicted of poisoning her husband, the famous painter Amyas Crale, and died in prison. However, before her death, Caroline left a letter proclaiming her innocence. Now, Carla wants Poirot to find out the truth.

Poirot agrees to take on the challenge, despite the case being cold. He examines the five key witnesses, whom he refers to as the "Five Little Pigs" (after the nursery rhyme), all of whom were present at the time of the murder.