The thing about Halloween Party is that it has a lot of the trappings of a good, solid Hercule Poirot mystery. The problem is that, again, this is one of the later books, where Christie drones on and on about things that aren't as relevant, and the book feels a little stale as a mystery.
The plot of the book revolves around a young girl who claims to have witnessed a murder, and is murdered at a Halloween Party. Ariadne Oliver just happens to be at this party, and of course, she gets Hercule Poirot involved. There are shades of her previous mysteries here, including and most notably Dead Man's Folly, which has a very similar premise, though - I think is a much more interesting read.
You'd think the idea of something being set at a Halloween Party would be spooky, but there isn't much atmosphere in this one - despite some rather unorthodox things going on (especially in the end.) And usually, I enjoy the dynamic of Oliver and Poirot, but this is the blandest these characters have ever felt, which feels like a bummer - because even in weaker novels, I usually have that to look forward to.
The one somewhat interesting thing, though, is just how 60s this book feels. The youths in particular feel very much a product of their time, and it's kind of amusing seeing an elderly Christie's take on such an iconic time period. It was the one aspect of the book I remember from reading it as a kid, and I think it's the only aspect that really will stick with me.
Overall, it's a somewhat bland and boring book, a somewhat pale shade of the mysteries Christie had once done so brilliantly. It's not terrible or bad, but it does feel like it's missing something.