Their magic tree house takes Jack and Annie back to a monastery in medieval Ireland, where they try to retrieve a lost book while being menaced by Viking raiders.
Eight-year-old Jack and his younger sister Annie use the magic treehouse to travel back to the Middle Ages, where they explore a castle and are helped by a mysterious knight.
After painstakingly handcrafting a replacement copy of a library book, a medieval monk tries to protect it from a hungry bear with a taste for literature. Includes historical note on illuminated manuscripts.