The novel was written originally as 11 or 12 short stories with the central theme of Poirot's search for these arch villains. All 4 are self-centred, and have world domination as their individual goal.
At first the identity of just two people are known, then the third is revealed, but Number 4 is like a chameleon, able to take on many disguises and then leave no lasting impression of his face. This 18 chapter novel focusses on 4 people who threaten world security with catastrophe and destruction. Poirot realises that he is being warned not to embark for South America. Plans are thrown into disarray by the uninvited guest who comes into Poirot's bedroom, collapses, and then dies. He had decided to surprise Hastings with his arrival. He arrives to find Poirot about to leave to sail to South America. Hastings returns from Argentina without warning Poirot that he is coming. I had forgotten the unusual structure of this novel. The book felt like it was a collection of short stories or four novellas melded together to make a complete book - which I found out that it was comprised of a selection of short stories! Also he uses other identities in his sleuthing.
It is a bit different in that the time span is longer than usual for Poirot. Little by little, Poirot learns more about each of the Big Four and is able to come up with a solution to stopping the plans of the Four: plans that will affect the governments of the world. This particular case runs over a number of months. One thing that makes it bearable is having Hastings involved to assist him. Much to Poirot’s dislike, but a major requirement, he finds himself travelling to various locations in order to learn more about each of his adversaries. Hurcule Poirot finds himself involved in a case of international proportions involving multiple murders planned by four very powerful figures a Chinese man, a French woman scientist, a wealthy American and a master of disguise, together known as the Big Four.