ITV's "Miss Marple" series exists of a good mix between Agatha Christie stories I know almost by heart, because I read the novels or they have been previously adapted for the screen ("The Body in the Library", "The Mirror Cracked from Side to Side", "Murder at the Vicarage"...), and stories that are entirely new to me. In case of "Greenshaw's Folly", it's even TWO entirely new stories. Apart from the titular short story, the film also incorporates elements of a Christie tale named "The Thumb Mark of St. Peter".
"Greenshaw's Folly" is a more than adequate and entertaining whodunit, but at the same time I also think it'll be the most forgettable of all twenty-thee episodes in the series. The plot, characters, red herrings and even the denouement (albeit still quite impossible to predict) all feel very familiar and derivative. Also, and I do realize this is very personal, the film demonstrates why Julia McKenzie is my least favorite of all the actresses that depicted Miss Marple. I love the character because she's supposed to be a clever, silent and inconspicuous spectator. McKenzie, much more than - say - Joan Hickson or Geraldine McEwen - really depicts her like an intrusive private detective.