Poirot investigates the brutal murder of an American heiress and the theft of a fabulous ruby on the Blue Train between Calais and Nice. After traveling on the Blue Train from Calais to Nice, Hercule Poirot is pressed into service to help solve the murder of heiress Ruth Kettering who is found savagely beaten in her compartment. She was the daughter of wealthy industrialist Rufus Van Alden and very much wanted a divorce. Both her husband and her lover were on the train but she had changed rooms with another passenger, Katherine Grey, so the question naturally arises as to whether she was the intended victim. Grey may also have had enemies as she had recently inherited a very large sum of money and greedy relatives had suddenly taken an interest in her. When an attempt is subsequently made on Grey's life, this appears to the case but Poirot methodically sifts through all of the clues to determine the motive and identify the killer.