The strange, maniacal blue owl known as the Striga has been rousted from the Great Ga'Hoole Tree. Nyra, leader of the vicious Pure Ones, is either dead or laying low in some distant land, leaving the tree finally at peace. As if fed by an invisible spring, learning and the lively arts flourish at the great tree and spread throughout the owl kingdoms.

But unbeknowst to the Guardians, in a long-empty cave deep in the Northern Kingdoms two ruthless villains join forces to conjure an ancient evil, an evil that will do their bidding and wreak havoc on the world.

When word of this growing malevolence reaches Coryn, Soren, and the Band, the young king knows he must do two things: First, he must return the ember to the Sacred Volcanoes, for the same subtle emanation from the Ember of Hoole that stimulates the quest for knowledge and invention at the great tree, also magnifies the powers of those who seek to destroy it and all for which it stands. Second, he must gather allies. Two armies grow. Gadfeathers, bears, dire wolves, and greenowls join with the Guardians, while on the other side ancient evil takes to the sky.