With her reluctant student, Arnold, the Friz slips through a mysterious trap door -- and ends up in the Middle Ages! There, they explore a medieval castle, meeting the people who live there and learning about everyday life within the castle walls. When the castle is threatened by invaders, Arnold and the Friz don chain mail to lead the defense. They repel the invaders just in the nick of time, then they joust and feast before returning to the 21st century. With their careful research and trademark wit, Cole and Degen introduce the many facets of medieval life.