The Big Four is a high-stakes mystery thriller that blends detective fiction with international espionage. When Hercule Poirot becomes aware of a shadowy organisation operating beyond national borders, he finds himself confronting a conspiracy unlike any he has faced before. Known only as "The Big Four," the group represents power, intellect and ruthless ambition on a global scale. As events unfold through a series of interlinked cases, Poirot and his associates are drawn into a battle of wits against an enemy that thrives on secrecy and manipulation. Disguises, false identities and sudden reversals blur the line between truth and illusion. This annotated edition pairs the original novel with structured chapter-by-chapter summary notes that clarify the episodic plot, track the evolving threat and illuminate Christie's experiment with thriller conventions during a formative period of her career. First published in 1927, The Big Four stands apart from Christie's village mysteries, offering a glimpse of her engagement with early twentieth-century spy fiction.