An ailing Poirot returns to Styles with Hastings nearly three decades after solving their first mystery together there in order to prevent an unscrupulous and ingenious serial killer from claiming more victims.

In an effort to recover his confidence after a rare professional failure, Poirot sets off for the Swiss Alps in pursuit of a missing maid.

Poirot investigates a strange and gruesome murder of an elderly psychiatrist. Ariadne Oliver is pressed to try and uncover the truth behind two decade-old deaths. They soon find out that their separate investigations are linked and work together to try and solve the mystery.

Ariadne Oliver is asked to devise a murder hunt for a Devon fête, but her sense of foreboding summons Poirot to the scene. Her fears are realized when, during the fête, the girl playing her murder victim winds up well and truly murdered.

With the world on the brink of WWII, there is more political turmoil when it appears that the famous Peace Party is in fact a cover for the Big Four.