"The Big Four" is a novel by Agatha Christie, published in 1927, and it features her famous detective, Hercule Poirot. In this book, Poirot is faced with a formidable adversary: a criminal organization known as the "Big Four", which is composed of four members from different parts of the world. The group is determined to carry out a series of crimes that will enable them to take over the world.

The novel begins with Poirot receiving a cryptic letter that alerts him to the existence of the "Big Four" and their nefarious plans. He teams up with a new ally, a Scotland Yard detective named Edward Catchpool, and they embark on a quest to stop the "Big Four" from carrying out their sinister schemes.

Poirot and Catchpool travel to various locations, including Paris and Argentina, to gather clues and unravel the complex web of the "Big Four" organization. Along the way, they encounter a number of dangerous and suspicious characters who may be involved with the "Big Four" or who may simply be trying to throw them off the trail. As Poirot and Catchpool work to solve the mystery, they encounter a number of twists and turns that keep the reader guessing until the very end. The novel is full of suspense and intrigue as the detectives race against time to stop the "Big Four" from carrying out their plans and wreaking havoc on the world.