Harry Potter and the Prisoner of Azkaban

As Harry Potter begins his third year as a student at the Hogwarts School for Witchcraft and Wizardry, he is disturbed to discover that Sirius Black, a wizard who may have been involved with the death of his parents, has escaped from Azkaban Prison. Black left behind two clues, both of which indicate that he is headed for Hogwarts—and Harry. As a result, the school is being watched over by the Dementors, a terrifying sect of black-cloaked guards from the Azkaban Prison—whose very presence at Hogwarts has a potentially life-threatening effect on Harry. Meanwhile, he gets acquainted with Professor Lupin, Hogwarts's new teacher of Defense Against the Dark Arts, but Harry also wonders about his new teacher's secretive relationship with Professor Snape.