Ruth Kettering is a rich young lady travelling on the Blue Train to the French Rivieria, in her belongings are priceless rubies known as Heart of Fire. Also travelling on the train is another young lady Katherine Grey recently inherited a large sum of money, the two ladies meet. Ruth Kettering is meeting her lover, but unbeknown to her, her estranged husband Derek is also on the same train. Also on the train is Mirelle the mistress of Derek, Ada Mason the maid and Hercule Poirot.
The next morning Ruth Kettering is found murdered and her jewels stolen. The dead girl`s father Rufus van Aldin and his secretary Major Knighton asks Poirot to help with investigation.
At first it is thought that both crimes were done by the same person but Poirot begins to think they are separate cases. Eventually the estranged husband Derek becomes prime suspect but Poirot is not so sure about this.
I very much enjoyed this novel and have said on numerous occasions that I love Agatha Christie books. I think I should read more about the author herself too.