A tragedy occurred aboard the luxurious express “Blue Train” traveling from London to the French Riviera. Ruth Kettering, the daughter of a famous American millionaire, was strangled in her own compartment, and all her precious jewels—including an exquisite ruby—disappeared. Traveling on the same train was the sea-loving great detective Hercule Poirot, who, of course, couldn’t stay aside from the investigation. Everyone around him is talking only about the mysterious jewel thief known as the Marquis, and they are completely sure that this crime is his doing. However, Poirot receives information that, just before the unfortunate woman was found dead, her husband Derek left her compartment…