The third novel in the series marks a significant shift in tone, introducing darker themes and revealing crucial details about the First Wizarding War and the history of Harry Potter's parents. After accidentally inflating his aunt, Marge Dursley, Harry flees Number 4, Privet Drive. He is picked up by the Knight Bus, an emergency transport for witches and wizards, and travels to the Leaky Cauldron. There, he is met by the Minister for Magic, Cornelius Fudge, who surprisingly does not punish him. Harry learns that a dangerous murderer, Sirius Black, has escaped from the wizarding prison, Azkaban, and is believed to be hunting him. On the Hogwarts Express, the train is stopped and boarded by Dementors, the ghoulish guards of Azkaban, who are searching for Black. One Dementor attacks Harry, causing him to faint before he is rescued by the new Defence Against the Dark Arts teacher, Professor Remus Lupin, who repels the creature with the Patronus Charm. At Hogwarts School of Witchcraft and Wizardry, security is tightened with Dementors patrolling the grounds. The school year introduces two new teachers: Lupin and Professor Sybill Trelawney, who teaches Divination. Rubeus Hagrid is appointed the new Care of Magical Creatures professor, but his first class goes awry when Draco Malfoy provokes a Hippogriff named Buckbeak, who injures him. Malfoy's father ensures Buckbeak is sentenced to death. Harry discovers that Sirius Black was his parents' best friend, his own godfather, and the one who allegedly betrayed James and Lily Potter to Lord Voldemort. Fueled by a desire for revenge, Harry tries to find Black. He is aided by a magical map of Hogwarts given to him by Fred and George Weasley: the Marauder's Map, which shows the location of everyone within the castle. Using the map, he sneaks into the nearby village of Hogsmeade. The climax occurs when Ron Weasley is dragged by a large black dog into a tunnel beneath the Whomping Willow. Harry and Hermione Granger follow, discovering the tunnel leads to the Shrieking Shack. The dog is revealed to be Sirius Black, who is an Animagus. Professor Lupin arrives and reveals the full truth: Black is innocent. The real traitor was another of their school friends, Peter Pettigrew, who has been hiding for twelve years in his Animagus form as Ron's pet rat, Scabbers. Severus Snape arrives and attempts to hand Black over to the Dementors, but Harry, Ron, and Hermione stun him. They force Pettigrew back into his human form and head back to the castle. However, the full moon appears, and Lupin, a werewolf, transforms. In the ensuing chaos, Pettigrew escapes. Black is captured by Dementors, who nearly perform the Dementor's Kiss on both him and Harry before a powerful Patronus drives them away. Following advice from Albus Dumbledore, Harry and Hermione use a Time-Turner, a device that allows for time travel, to go back three hours. They successfully rescue Buckbeak from execution and then save Sirius and Harry's past self from the Dementors, with Harry realizing he was the one who cast the powerful stag Patronus. Sirius escapes on Buckbeak, a fugitive but a free man. Professor Lupin resigns after his status as a werewolf is revealed.
Harry Potter and the Prisoner of Azkaban introduces several critical elements to the wizarding world.
This book is a turning point in the series. It moves away from the more mystery-oriented plots of the first two books and delves deeply into the history of the Potter family and the First Wizarding War. The introduction of Sirius Black and Remus Lupin provides Harry with a direct link to his parents and a new, loving support system outside of Hogwarts. The revelation that a trusted friend, Peter Pettigrew, was the true betrayer complicates the narrative's understanding of good and evil. The book's darker tone and more complex emotional themes, particularly concerning fear and despair personified by the Dementors, set the stage for the escalating conflicts in the subsequent novels.