During the 9th century, in a desolate valley in Ireland that shelters the fortress abbey of Kells, lives Brendan, a twelve-year-old monk. With his other brothers, Brendan participates in the construction of a surrounding wall to keep the abbey at bay from regular Viking assaults.

His encounter with Brother Aidan, a renowned master illuminator and "guardian" of a book of fabulous illuminations that is not yet complete, will lead him on fantastic adventures. Aidan initiates Brendan into the art of illuminations, for which the young boy reveals a prodigious talent.

To finish the book and defy his own fears, Brendan leaves the abbey for the first time and enters the enchanted forest where dangerous mythical creatures hide and wait for him. It's here that he meets Aisling, the young child-wolf who will help along the way.

Will Brendan succeed in his mission, despite the Viking hordes who approach? Will he be able to prove that art is the best fortification against barbarians?