After accidentally inflating his aunt Marge, Harry leaves the Dursleys, and catches a ride on the street on a magical bus called The Knight Bus. Before he begins his way back to Hogwarts, he learns of Sirius Black, a mass murderer, believed to have killed thirteen people with a single curse. Black has escaped from the wizard prison, Azkaban. Meanwhile, upon meeting the Weasleys, Harry learns that, despite accidentally using magic (which is illegal), he has not been expelled. Mr. Weasley tells Mrs. Weasley of Sirius Black's motive to kill Harry when Harry overhears this he is told not to go looking for Black. Later in the book, Harry soon realizes why. 