They live in Little Tunks, with their father, Casper, who paints dethroned royalty for a living. Classically, their mother is missing or dead, and no one seems to know which. Fortunately, nothing else about this book is very classical. When their father leaves suddenly and the Hardscrabble children are left alone in London... well, a lot happens. Castles and castle follies, missing sultans, bikes in the air, and perhaps a misshapen freak of a boy haunting the woods... and more.
While the plot (or plots, really) is definitely complex enough to keep your attention, it's really the characters that stand out. In fact, all three main characters were amazing. If that's not a feat, I don't know what is. There's silent, mysterious Otto (my favorite), with his sign language and penchant for animals and perhaps a few secrets up his sleeve. Lucia, who could have easily turned out horrible, was perfect as a business-like but sweet sister (look at her picture on the cover - that's pretty much her). Max was cute and believably intelligent without being too over-the-top. And they all work perfectly together. The other characters, though perhaps not as fully developed, are almost as engaging.
There are several plot points at work in The Kneebone Boy. Curiously, the titular entity isn't the main point - or is he...? Then there is of course the missing mother, the castle folly where the Hardscrabbles stay, even their relationships with each other (Lucia and Otto - one of the most priceless pictures of sibling relations I've ever read).
Sadly, Kneebone Boy is one of those books that reminds you not to celebrate over a book until it's over. The ending wasn't disappointing, per se; it was just... easy. It was so easy that I had to read through it a couple times to make sure I hadn't missed something. Yes, it all makes sense; but it was drab compared to the caliber of the previous adventures. In the words of an Amazon reviewer, "...All Is Explained. And this is the kind of book where you kind of wish it weren't."
Don't look for a happily easy ending, either. It's really rather sad once you get past the shock of not being shocked. But it fits somehow. And honestly, the colossal fun of this American-written-British-quirk book is worth the ending.