The Mystery of the Blue Train is a detective novel written by Agatha Christie. The book is one of Christie's Hercule Poirot mysteries. The story is set in France and concerns the murder of an American heiress on Le Train Bleu, the titular "Blue Train". Poirot boards Le Train Bleu, bound for the French Riviera. So does Katherine Grey, who is having her first winter out of England, after recently receiving a relatively large inheritance. On board the train Grey meets Ruth Kettering, an American heiress leaving her unhappy marriage to meet her lover. The next morning, though, Ruth is found dead in her compartment, a victim of strangulation.