Jack is a shy but confident bookworm, and his sister Annie can quickly make friends with whoever she wants, both humans and animals. One day, while they were playing together in a forest, they unexpectedly stumble upon a wooden treehouse located in this forest. This magical treehouse allows people to travel across time and space whenever they liked. 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.
