Harry Potter and the Prisoner of Azkaban

Harry's summer at the Dursleys' abruptly comes to an end when Aunt Marge has an unfortunate encounter with magic. He uses the Knight Bus to flee to the Leaky Cauldron, where he stays until the beginning of the new term and learns that the mass murderer Sirius Black, who has managed to escape from Azkaban, is actually looking for him. Back at school, Harry quickly notices that some things have changed. Because of Black's escape, Hogwarts is guarded by Dementors whose presence influence him strongly. On a forbidden excursion to Hogsmeade, Harry overhears that Black is responsible for the death of his parents.
After having been afraid at first his hate finally is stronger than his fear Black. Even though he has promised not to put himself into danger, he is soon offered an opportunity of taking revenge.

In the end everything turns out differently and Harry has to decide whom to trust.