Hastings finally manages to surprise Poirot by showing up on his doorstep. Also there? A mysterious stranger who collapses more or less in the doorway while holding a slip of paper covered in the number four. From there, Poirot (and Hastings) are plunged into a mysterious world where four people are known to be running a large criminal organization. A series of small mysteries add up to one big showdown with the introduction of Poirot's twin brother, Aristotle.
Poor Hastings isn't treated very well in this book. He keeps thinking he is being so intelligent but he is just a pawn, not only of the Big 4 but of Poirot as well.
Three stars
This book came out January 27, 1927
Follows The Murder of Roger Ackroyd
Followed by The Mystery of the Blue Train
Borrowed as hard copy from library
Opinions are my own