I found this the most complicated Christie I have read so far, and I think it's the one I've enjoyed least. I really found it very difficult to follow.
The story begins with Hastings coming back from Argentina for a holiday. He pays a surprise visit to his friend Hercule Poirot. Unfortunately he finds Poirot just about to leave on a trip to South America on a case. As Hastings and Poirot are talking, they hear a noise from one of the inner rooms of Poirot's apartment;
The door swung slowly open. Framed in the doorway stood a man. He was coated from head to foot with dust and mud; his face was thin and emaciated. He stared at us for a moment, and then swayed and fell.
This man tells Poirot of the 'Big Four.' This appears to be an international group of villains, and Poirot is fascinated. He realises that the case in South America is a ruse to get him out of the way and he stays in England and starts to investigate the Big Four.
This is the first Poirot mystery I've read where Poirot seems to be flummoxed at times. I was flummoxed all the time. I'm not writing much because I was so confused that I really can't remember the details of the story.
I read this as part of the Agatha Christie Reading Challenge.