Poirot tells a surprised Hastings he's decided to retire and suggests they take a holiday in the Lake District. On the way, Hastings befriends a young woman who is missing a collection of valuable miniatures. The case involves a famous writer and a beautiful woman in a sports car, and to the relief of Hastings Poirot's thoughts of retirement soon seem far away.

A glamorous nightclub singer turned spy has stolen the US Navy's plans for a new submarine, and an FBI agent called Burt is in London to catch her. Burt dismisses Poirot as a gumshoe, and Poirot is likewise not impressed by the self-important G-man. Poirot is drawn into the case when it appears that the submarine plans will fall into the hands of the Fascist government in Italy unless they are recovered. When Poirot meets a young couple who've got an amazing bargain on a prime apartment, he sees a connection with the spy case.

Marie Marvelle, a Belgian film star, receives threatening letters from a chinaman which demand her famous diamond, the Western Star. Marie visits her old friend Hercule Poirot in London, seeking his help, and Poirot tackles the case with relish.