2024 Reprint of the 1927 Edition. In this mystery, created from a reworked collection of short stories, Poirot enters the world of international espionage. The original stories were published in 1924 and it was in 1926 that Agatha Christie, in need of a new book, gathered them together with the help of her brother-in-law and submitted them to her publisher.

Summary: An unexpected visitor called Mayerling comes in through Hercule Poirot's bedroom and collapses on the floor. The only clue to what he wants is his repeating Poirot's name and address and writing the number 4, many times. When Hastings jokingly calls it "The Mystery of the Big Four," the man begins speaking about an international crime cartel of that name. He describes the four leaders: Number 1 is a Chinese political mastermind named Li Chang Yen; Number 2 is probably American; Number 3 is a Frenchwoman; and Number 4 is known only as "the Destroyer." The man dies soon after Poirot and Hastings go off on the trail of the Big Four. It is up to Poirot to discover and expose this international plot aimed at world domination.