S03E09 - The Plymouth Express

A young naval officer undertakes a train journey on the Plymouth express and finds the dead body of a woman underneath one of the seats in his carriage. The woman is later identified as Flossie Halliday, latterly the Honourable Mrs. Rupert Carrington, the daughter of an American steel magnate who asks Poirot to take the case on.

Poirot knows of the woman and saw sight of her once in Paris. Some time ago she was caught up with an adventurer called Count de la Rochefour but her father took her back to America to get her away from the Count. She later married Rupert Carrington who was to prove just as unsuitable a husband, being a gambling addict and deep in debt. They soon drifted apart and were about to announce a legal separation.