Poirot is dining alone in a London restaurant and strikes up a friendship with Katherine Grey. She has just inherited a lot of money and is about to travel to visit relations in the South of France, but is nervous. Poirot offers to go with her on the Blue Train.

Apart from Poirot and Katherine, others on the train include the Tamplin family (Katherine's cousins), millionaire Rufus van Aldin and his daughter Ruth Kettering, Ruth's penniliess husband Derek, and her boyfriend - and Ruth has a very valuable ruby with her called 'The Heart of Fire'. And then Ruth gets killed on the train in a gruesome and disfiguring attack, and at the same time her ruby is stolen. And after the arrival of Katherine and Poirot at the Tamplins' villa, Katherine is also attacked. Poirot finds himself dealing with several crimes at once.