Wealthy Australian business man Mr Halliday tries to engage Poirot to look into Comte de la Rochefour, a dubious French man interested in his daughter Flossy. Poirot has no stomach for investigating domestic troubles, but he's needed for something more serious.
I was eleven when this episode went out and it left its mark, the actual murder scene is perhaps one of the darkest moments in the show's history, it lingered longer then usual, and was more graphic then usual, even the description was bleak. (tame by today's standards I know.)
As always with the series it's a gorgeous looking episode, the sets, buildings, clothes etc are perfectly in character, the scenes at the train station particularly look great.
It's a fairly simple mystery, but the strength of it lays in the impact of Flossy's death, her father is grief stricken and even Poirot is hit hard by it. In some episodes murder seems almost casual, not the case with the Plymouth Express, once she's found the episode takes on a different feel.
I actually think it's a goody. 8/10