The Great Tree of Ga'Hoole is in turmoil. The night guards understand that the Pure Ones, led by their leader Kludd, are planning something. Ezilrib decides to send spies to the Academy of Sant'Egolius, where grains of magical power are stored. It is they who must help the owls of the Great Tree defeat their enemies. At this time, a spy appears on the tree itself. In some incredible way, the enemies manage to learn about all the plans of the Night Guards almost on the day they are adopted. Soren begins to suspect his sister Eglantine of falling under the influence of the Pure Ones. Preparing for the final battle with their enemies, the pack goes to the Northern Kingdoms to learn the art of warfare. The future of the owl kingdoms is in danger. The upcoming battle will require boundless heroism, perseverance, courage and, of course, wisdom from all the owls of the Great Tree.