It's been years since I read the Agatha Christie canon -- and I read every one -- but I don't remember all of them well enough to do a comparison with the book to the movie.
"The Pale Horse" is a Christie book that does not feature Jane Marple, and yet, here she is. Before I go on, I need to say one thing. I don't know why the producers of this series decided to change Christie's stories. However, Acorn Media owns something like 67% of the Christie estate, and apparently, in the negotiations, Christie's relative, Mathew Pritchard, who heads the estate, put no restrictions on the sale. So all of this is okay with the estate. In other words - if they don't care, why should we?
In this story, Miss Marple receives a letter from an old friend, Father Gorman, with a list of names and a Biblical quote. The same day she receives the letter, she reads that Gorman was murdered right after attending to a dying woman.
Miss Marple is right on it, traveling to the area where Mrs. Davis lived and checking out her room. There, she finds the same list. On tracking down the names, she finds out that all of the people have recently died. She also discovers a reference to the Pale Horse Inn, which ties in with the Bible quote. She stays at the inn and begins to investigate.
I found this particular movie a little confusing and hard to follow. The subplots were not woven in together as well as they should have been, making it a little disjointed. I did like Julia McKenzie, who is a fine actress.
I wouldn't say this is one of the best of the series, but it's okay.