It is Harry Potter's third year at Hogwart's School of Witchcraft and Wizardry. The book starts out with Harry getting some revenge on his Uncle Vernon's sister, who has come for a visit and has been berating him for years. When she begins to viciously insult his parents, however, Harry finally snaps and puts a spell on her. This incident causes him to flee the Dursley's in anger and frustration over the years of bad treatment in their home.

As he heads off down the street, he is dreading his expulsion from Hogwart's for performing underage magic. Much to his surprise, he is not expelled and the Minister of Magic himself appears at the Leaky Cauldron to tell him that he is not in trouble. Instead, he must stay indoors. Sirius Black, a murderer, had escaped from Azkaban the wizard's prison. He is very dangerous and...