The Viking boy Ubbe loses his mother when the dreaded Fenris wolf attacks his village. Four years later, the god Loki appears in the form of a raven. He persuades Ubbe and the spellmaster Tyra to join in a hunt for the beast.
On the journey, they encounters both dangers and obstacles. The worst comes in the form of a strange and combative Viking, Blåtann jarl. After an exchange of insults, Ubbe is challenged to a duel - a deadly battle called Fimbulvinter.