My wife has been going through and reading all the Hercule Poirot stories from Ms. Christie, and we picked this one up to read together, so we were a bit surprised to find that M. Poirot was noticeably absent. As well as being a traditional mystery, the book is also a character analysis: when it turns out that the man convicted of the murder was innocent, it means that another of the family must have been the murderer, and the family members react to this information in their various ways. But the traditional gathering-together-of-the-suspects-to-reveal-the-killer also takes place, with one Arthur Calgary playing the role of detective. He's not a character I'm aware of from any other books, and since he is occupied as a scientist rather than a detective or policeman, I suspect he is a one-off character brought in to keep the focus of attention on the family rather than M. Poirot's mustaches. But he's not the only sleuth; Philip Durrant, a member of the family, also is digging around, while the local police are trying to rectify their original error.
The characters all have personalities that are a bit unusual; not surprising considering their backgrounds as war orphans. The majority of the first three-quarters or so of the book advance the plot very slowly, as the characters are developed and we watch them bouncing off each other. But the action picks up at the end, and the last part of the book moves along with the pacing of a Robert Ludlum novel.
But there are a couple of weak points, which I can't discuss without revealing quite a bit of the plot; see below. So it's not really one of Ms. Christie's stronger works. One bad apple in a barrel of really tasty ones is only to be expected, I suppose.
WARNING: Spoilers below. I'm not sure if it's all that important to place that warning on a review of a 50-year-old book, but hey, there it is if it's necessary :)
I didn't buy into the ending. It revolves around a woman falling in love and being betrayed by a man whose diapers she changed. IMO it's not likely that that would occur; if you grow up with someone you will more or less know how they will react in a given situation and you would know whether or not they are lying.
A stabbing occurs near the end of the book. The plot dictates that the stabbed woman (a) does not realize she was stabbed, and (b) walks 50 or 100 feet before fainting. A doctor justifies to us that this was possible, but is it really? I have a hard time believing it.