Bound for the Riviera, detective Hercule Poirot has boarded Le Train Bleu, an elegant, leisurely means of travel, free of intrigue. Then he meets Ruth Kettering. The American heiress, bailing out of a doomed marriage, is en route to reconcile with her former lover. But by morning, her private affairs are made public when she is found murdered in her luxury compartment. The rumour of a strange man loitering in the victim's shadow is all Poirot has to go on, until Mrs. Kettering's secret life begins to unfold.