Tragedy struck on the luxurious Blue Train, an express train from London to the French Riviera. Ruth Kettering, the daughter of a famous American millionaire, was strangled in her compartment, and all her jewelry, including a stunning ruby, disappeared. The great detective Hercule Poirot was also on the train, traveling to the seaside for a seaside vacation. Naturally, he couldn't stay away from the investigation. Everyone around him was talking about the mysterious jewel thief known as the Marquis, and they were absolutely certain he was behind the crime. However, Poirot received information that shortly before the unfortunate woman was discovered dead, Ruth's husband, Derek, had emerged from her compartment.