In his sixth adventure, Hercule Poirot is on board the famous Blue Train from Calais to Nice. He encounters four different people and groups of people who are all after the Heart of Fire, a spectacular ruby purchased by American tycoon Rufus Van Aldin and presented to his daughter Ruth. Ruth is unhappily married to British aristocrat Derek Kettering, a richly layered character about whom the reader is still trying to decide if he is hero or villain up to the final chapter. This novel is filled with exciting characters: Mirelle, the exotic dancer with a passion for Derek; Armand de la Roche, the attractive but notorious swindler; and most refreshing of all is Katherine Grey, one of Mrs. Christie's best heroines.
When Ruth Van Aldin Kettering is found murdered on the Blue Train en route to her annual winter trip to the French Riviera, it is up to Hercule Poirot to discover if she was murdered because the famous jewel was in her possession or was she murdered by her husband or his mistress or was there yet another sinister motive.
This excellent tours de force is a landmark book for Christie fans because from this point until sometime in the late 60's every novel she published was brilliantly plotted and never failed to challenge the mystery reader.
Agatha Christie was known for experimenting with plots in short stories before developing them more fully in novels. The Mystery of the Blue Train is a prime example of this, so you might wish to go back and read her earlier short story "The Mystery of the Plymouth Express" if you enjoyed this one.