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.
Poirot reunites with his old friends Hastings, Japp, and Miss Lemon, and they are pitted against a dangerous group of dissidents as the threat of impending world war looms.
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.
In the penultimate film Poirot falls into a deep depression, after his egotism plays a part in the murder of a society girl. It is the desperate plea of a lonely chauffeur to find his missing soul mate, which eventually motivates Poirot to get back to work. His quest takes him to the Swiss Alps where he stumbles on clues, which may lead to a very personal showdown.
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.