The Big Four

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.

From here, the novel becomes a series of loosely connected short stories.

• Poirot and Hastings visit an informant, John Ingles, to ask him about the Big Four. Ingles shows Poirot a note from a fisherman who asked him for a few hundred pounds to hide himself from the Big Four. Poirot, Hastings and Ingles discover that the man who wrote the note, a Mr Jonathan Whalley, has been murdered by Number 4.
• Poirot discovers that the Big Four have produced a form of wireless energy capable of focusing a beam of great intensity on any spot. A British scientist called Halliday was near success on this same concept when he was kidnapped while at a conference in France. Poirot and Hastings visit the notable French scientist Madame Olivier and discover she is Number 3.
• Poirot discovers that American millionaire Abe Ryland is Number 2 and sends Hastings to spy on him.
• Poirot investigates the death of a Mr Paynter in Worcestershire. Before his death, Paynter had written in ink "yellow jasmine" on his newspaper, and attempted to draw a number 4. Poirot reveals that Paynter's attending physician, a Doctor Quentin, was in fact Number 4, who gave Paynter an injection of yellow jasmine.
• A month later, Japp informs Poirot of another mysterious death—that of chess grandmaster Gilmour Wilson, who died from heart failure while participating in a match with Russian refugee Doctor Savaronoff. Poirot deduces that the real Savaronoff died in Russia and that Number Four impersonated him, killing Wilson in order to preserve his cover.
• Hastings is kidnapped by the Big Four and Poirot saves him.
• Poirot identifies Number 4 as an obscure character actor called Claude Darrell. A former girlfriend reveals Darrell's identifying quirks.
• In order to track the Big Four in secret, Poirot stages his own death. He and Hastings travel to the Big Four's mountain hideout in Italy and are taken captive. Poirot reveals that he is not Hercule Poirot, but his twin brother, Achille. He tells the Big Four that the mountain has been cordoned off and the police are on the scene. Three of the Big Four are killed in an explosion just as the police raid the hideout. The other is reported to have committed suicide. Back home, Poirot reveals Achille Poirot did not exist – it was Hercule Poirot in disguise all along. He laments that all his other cases will seem boring and tame compared to this case. The novel ends with Hastings returning to Argentina and Poirot considering retirement.