It's a story about four siblings who have been sent to spend the summer with a great aunt and uncle while their parents are working out the details of their divorce. All of the kids are miserable at being removed from their home, friends, and neighborhood. And they are sooo bored. And then certain elements creep into the story which seem to indicate that this will either be a fantasy or a book with supernatural bits. But alas! It all turns out to be merely psychological.
And I was kind of repulsed by the book's "lesson" which was that kids must stick together because adults cannot be depended on for anything. Family identity has shrunk to include kids only. As Nels tells his younger brother on the last page, "All us kids have to to stay together, that's the big thing. We've got to promise each other. If we stick together, then whatever happens outside -- whatever the grown-ups do -- it won't matter so much D'you see? We'll still be us."