 Morgan le Fay is accidentally transformed into a mouse named Peanut by Merlin (echoing the plot from books 5–8), and Jack and Annie have to find four magic medallions to rescue her.They visit the late Cretaceous period with dinosaurs (from book 1), a medieval castle (from book 2), the town of Pompeii (from book 13) and a pirate ship (from book 4).At the end, Peanut is once again turned into Morgan.