Harry Potter's third year at Hogwarts begins under a shadow. Sirius Black, convicted of betraying Harry's parents to Lord Voldemort and of killing thirteen people with a single curse, has escaped from Azkaban — the first person ever to do so. The Ministry of Magic believes Black is coming for Harry. The school is placed under the guard of the dementors, soul-draining prison wardens whose effect on Harry is more severe than on anyone else, plunging him into traumatic flashbacks every time they appear.
Amid this, Harry finds an unexpected mentor in the new Defence Against the Dark Arts teacher, Professor Lupin, whose calm competence and personal connection to Harry's past make him the most significant adult figure Harry has encountered since Hagrid. A stolen Marauder's Map, a time-turner, and a truth that contradicts everything Harry has been told about his parents' deaths combine to make this the most structurally inventive instalment in the series.
Widely regarded as the moment the Harry Potter novels found their full emotional and thematic range, Prisoner of Azkaban marks the shift from middle-grade adventure to something darker, more morally complex, and more deeply concerned with justice, loyalty, and the way the past refuses to stay buried.