American millionaire Rufus Van Aldin has only one weakness: his daughter Ruth. She, on the other hand, has two weaknesses: jewels and dangerous men. The paternal gift, the Ruby Flame Heart, will not bring her happiness... Catherine Grey is also traveling on the Blue Train, and she was the last to see the victim alive. From her testimony, Hercule Poirot begins his work. Who murdered the millionaire's daughter? Where is the Flame Heart? To whom will Catherine Grey give her heart? Fortunately - as he himself says in the novel - "Papa Poirot always laughs last."
