Court politics lead to Toby, 12, and Alex, 10, the sons of Sir Bobboli (who is largely running the kingdom since the King is ill) being named whipping boy for the next year so they must endure the punishments imposed for nasty practical joker Prince Baddkind's bad deeds. The resourceful and imaginative brothers work at making the Prince pay in kind for each indignity or pain they endure. They also teach the persons who impose the punishments on them to be careful about exactly what they say and do. At that same time the king's two banned nephews arrive incognito, hoping to use the quirks of the Book of Rules that govern the kingdom to block the trouble-making heir apparent Baddkind and reclaim the crown for themselves. The Bobboli boys, described by their father as "ever larger two-legged mice" for the way they get around the whole castle, stumble on this plot and must find ways to thwart it. In the process they witness several other events that no one was supposed to see and must eventually figure out how to make the facts of two thefts known so they can be dealt with since the Book of Rules says they are too young for their testimony to be accepted by the officials who would have to take action against those men.