The story concerns a plane-load of adolescent and pre-adolescent boys who crash land on a desert island during a war. They appear to be British and the war could be supposed to be WWII, but this is never made explicit.
It's been a while since I read it, but I'm fairly sure it's implied the war is nuclear. Certainly the aeroplane they are flying in has some sort of detachable 'passenger tube' (which is jettisoned, with them in, and is why they end up alone on the island), which doesn't fit with any aircraft during WWII (or indeed since). I think the intention was that it was some unspecified near-future war of the type that was feared at the time.
Quite so - what I was getting at was the authors' attempt to tell a story about characters in isolation, without explicit reference to social or historical placement.
The plot sets up a situation in which the characters are removed from all social constraints, and explores what happens to their society. The results are not pretty as some of the boys fall into the most savage and inhumane habits.
WG was concerned with deep motivating factors of human behaviour, and this startling book proposes ugly truths about what some people might do to get what they want - when they are allowed to.