Guardians of Ga'Hoole, Book Five

During a time of war, Soren's sister Eglantine unwittingly becomes involved with Kludd, leader of the villainous Pure Ones, after being brainwashed by an infiltrator. As her strange behavior raises suspicion, Soren and his friends work to uncover the truth and help her recover. Meanwhile, Kludd's ongoing battle with the Guardians of Ga'Hoole continues, culminating in a victory for the Guardians, though the conflict between Kludd and Soren persists.