 Heiress Ruth Kettering, daughter of American billionaire Rufus Van Aldin, is married to what her father believe to be a scoundrel, and has a lover that her husband believes to be another scoundrel. She is also the owner of a world-famous ruby. When she takes the Train Bleu to Nice, she is not alone. Her husband, her lover, her maid, a recently rich young woman with a past, and the money-grabbing distant relatives of the young woman are all aboard, and maybe more. But she couldn t have been alone when she was brutally murdered and the ruby stolen, no matter what everyone says. Unfortunately for the murderer, Hercule Poirot was also aboard the Train Bleu.