The early Middle Ages were also known as the 'Dark Ages'. People had to work like dogs to feed and clothe themselves. To save a lost story, Jack and Annie have to travel to Ireland at this difficult time. With the help of an Irish monk, they finally get the book they need.

However, cold-blooded Viking raiders are attacking the coastal villages in Ireland and chasing Jack and Annie. Can they escape this time? Morgan, the magical librarian of the tree house told them an ancient story could save them. How could it stop blood-thirsty Vikings from killing them?