Poirot becomes involved in investigating the brutal murder of American heiress Ruth Kettering and the disappearance of her fabulous ruby known as the 'Heart of Fire' while he is on the Blue Train traveling between Calais and Nice. Who is to blame for this? To put together the pieces of the tragic puzzle, Poirot interviews the girl's father, her maid, her estranged husband, her eccentric cousins, and a host of other peculiar passengers.