The Striga, former dragon owl from the Middle Kingdom beyond the Unnamed Sea, has come to stay at the great tree. He has earned the trust of all by saving Bell, Soren's owlet, from Nyra, and he grows daily closer to the young king Coryn, with whom he seems to share a strange bond. The Striga senses the power of the ember hidden in Bubo's forge and draws it closer. As his power waxes, he accuses the Band of treason and produces flimsy evidence to support his abominable claim. And so the Band is exiled, strengthening the Striga's hold over Coryn.

The Band seeks refuge in distant forests and enlists the help of old friends, but every action they take is construed as further treason, and agents of the Striga - former Guardians - hunt them mercilessly, until for safety sake the band itself must separate. They devise as system of messages and alarms and so communicate despite their isolation. Old friends, loyal family and allies help devise a plan to shake the hold the Striga has on Coryn and to free the young king and the tree from this new, insidious tyranny. It will take courage, daring and wisdom, but the Band will meet the new challenge - for the sake of the king, and the Great Ga'Hoole Tree.