This is a tale of international intrigue and espionage, therefore opening up the possibility of more spy fiction from Christie.
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 and Poirot and Hastings go off on the trail of the Big Four.