Charlie and Tilda Hacker discover a secret attic room above their parents' antiques shop, which contains a key to time travel. They embark on a journey back over a thousand years to Viking York, armed with a cryptic message from Professor Howe. To find him, they must solve a series of riddles while navigating the challenges of Viking life. The story immerses readers in the world of Vikings, offering insights into their culture, language, and daily life.