Plot summary from Wikipedia:
The story is set in the ninth century. Twelve-year-old Brendan is educated by his uncle, Abbot Cellach, who holds a firm grip on his nephew and expects him to follow in his footsteps. One day, Brendan meets Brother Aidan, a master illustrator who shows him the beauty of art and stimulates his creativity and fantasy. Finally, Brendan decides to break free in search of his dream: completing the valuable Book of Kells. On his journey through the forest, he has to face his biggest fears.