Like Murder Most Foul (1964), this movie was adapted from a Poirot novel, not a Miss Marple novel.
The world première took place at a church garden party in rural Cheshire, England.
When reporting the second murder to the police on the phone, Miss Marple uses the words "Murder most foul", which also is the title of one of the following Miss Marple movies.
Miss Marple refers to "The Ninth Life" by Agatha Christie. This was an in-joke; her creator wrote no such book.
Miss J.T.V. Marple earned her Junior Silver Spurs at Brockbrook in 1910; she retained her mother's vintage 1885 Broadbreech side-saddle.