Right before Christmas Harry discovers that Sirius Black betrayed his parents to Voldemort, knowledge that fills him with rage. Fortunately, Harry forms a close relationship with the new Defense Against the Dark Arts teacher, Lupin, and Lupin works with him to learn the Patronus Charm. To add to Harry's misery, his horrific experiences as a baby mean that the dementors that guard the school affect him more than other students and he hears Lily screaming whenever they're close to him. Harry himself struggles to forgive Hermione when she tells McGonagall about the mysterious Firebolt broomstick he receives for Christmas, which results in the broom being confiscated. He finds himself caught between Ron and Hermione for much of the school year, as Hermione's cat Crookshanks makes constant attempts at the life of Ron's rat, Scabbers. Harry sees a hulking dog when he first runs away from the Dursleys and Professor Trelawney makes Harry concerned when she continually sees the Grim, a dog-shaped death omen, in his tealeaves and crystal ball. Third-years at Hogwarts get to take elective classes and Harry chooses to take Care of Magical Creatures, where he does his best to make Hagrid's lessons successes, and Divination, which proves trying. He inadvertently does magic and is surprised when he doesn't get expelled for his transgression, but he discovers that this is because the escaped criminal Sirius Black is on the loose and wants to kill him.

When Vernon's sister Marge visits, Harry loses his temper because she insults his parents. Because the Dursleys are muggles and hate magic, Harry's summers are horrendous. He lives with his Aunt Petunia and Uncle Vernon during the summer.

The protagonist of the novel a thirteen-year-old wizard who attends Hogwarts School for Witchcraft and Wizardry.

Azkaban