When ten-year-old Pidge finds the crumbling pages of an old manuscript in a second-hand bookshop in Galway, he unwittingly releases the serpent Olc-Glas--and the forces of good and evil gather to do battle. The Mórrígan, Goddess of Death and Destruction, has set her evil heart on gaining Olc-Glas and adding its poison to her own, thereby casting her shadow over the world.