It's early-ninth-century Ireland, and young, flame-haired Brendan (voice of Evan McGuire) is agog over the arrival of Iona refugee Aidan (Mick Lally) and his white cat, Pangur Bán, at the Abbey of Kells, since Aidan has brought the beginnings of the Book of St. Columba and means to finish it in the scriptorium at Kells. Brendan's uncle, Abbot Cellach (Brendan Gleeson), has no time for such nonsense, however — he's obsessed with building the walls that will protect Kells from the Vikings who've already sacked Iona.
So it falls to Brendan and Pangur Bán to enter the wild wood and, with the help of fairy girl Aisling (Christen Mooney), secure from human-sacrifice deity Crom Cruach the crystal Aidan requires to complete what we know as the Book of Kells. But it's the way the visuals tap into mediæval Celtic art that elevates this Oscar-nominated animation from Tomm Moore and Nora Twomey — from the almost abstract black wolves to the Eisenstein-like Vikings to the stylized green trees (which surround the abbey like the shields of Malcolm's Birnam Wood army) to the fantasy sequence where Brendan uses chalk to corral a writhing dragon within the book's design.
And Pangur Bán — the subject of an actual ninth-century Irish poem — has enough personality to fill a movie of his own.