Millie, Sanchez, and the gang are headed back to school, but a plane crash and an unsteerable raft quickly land them in deep waters. Rescued by a mad librarian, the students of Ribblestrop find themselves on a dangerous quest across the wilds of Ribblemoor. Can they uncover the ancient mysteries of the secret tribe that lives there? Will they survive the dangers of Lightning Tor? Inspector Cuthberson is right behind them, and he's determined to destroy the school--for good. Will Ribblestrop close down forever, or can the students keep the doors open for another zany semester?