A woman is murdered (Jane Seymour, so she's not here long). Was she killed by the man who died in prison for the crime, or one of her extended family? It's a case for Miss Marple.
Actually, it isn't. Miss Marple wasn't in this novel.
I haven't read the book so I can't speak to this episode's fidelity (well, Miss Marple's in it so that's one strike against fidelity from the get-go). But if they hadn't shoehorned Marple into some of these one-off novels casual Christie readers might never have heard of them. Which, in this case, doesn't seem to be a tragedy; though some other non-Poirot, non-Marple titles deserve to be better read.
When the novel came out several reviewers pointed out its lack of focus. I usually ignore "professional" reviewers but that seems to be the case with this episode. They also pointed out its lack of fun. That's a mood this episode retains. It rains a lot in this episode. That's the general mood.
At least, Julian Rhind-Tutt provides some welcome comic relief as Arthur Calgary, the novel's original crime-solver.
BTW, in this episode one character calls Miss Marple "benign." I think that's the problem many of us have with Geraldine's Miss Marple. Christie's real Miss Marple was called a lot of things, but never benign.