Fourteen-year-old Matilda, an apprentice bonesetter and practitioner of medicine in a village in medieval England, tries to reconcile the various aspects of her life, both spiritual and practical.

As Crispin tries to fulfill Bear's dream of moving to Iceland, he must leave Troth behind at a convent that needs a healer but, after falling in with thieves posing as musicians, he makes a new friend, Owen, and together they continue the arduous journey.

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.