Forced to spend the summer of his thirteenth birthday stuck with the Dursleys on Privet Drive, Harry Potter is counting down the days until he can return to Hogwarts. After accidentally causing his horrible Aunt Marge to inflate like a balloon one evening near the end of term, Harry flees Privet Drive and rents a room at The Leaky Cauldron for the remainder of the holiday. The night before boarding the Hogwarts Express with friends Ron and Hermione, Harry learns that an infamous dark wizard, Sirius Black, has escaped from Azkaban (a wizard’s prison) in order to find and murder Harry.
Harry’s third year gets off to a rough start. Professor Trelawney, the divination teacher, routinely predicts Harry’s early death to the class. Hagrid, recently appointed to teach Care of Magical Creatures, runs into trouble with the ministry after Draco Malfoy reports that Hagrid’s hippogriff, Buckbeak, is rogue and dangerous. The presence of dementors–Azkaban guards who feed on human emotion–surrounding the school for security only make Harry feel worse. Dementors have an unusual effect on Harry, triggering vivid recollections of his parents’ deaths. When the dementors storm the Quidditch Pitch and cause Harry to lose a match for Gryffindor, he persuades new Defense Against the Dark Arts teacher, Remus Lupin, to give him “anti-dementor lessons.” Lupin begins to teach Harry a patronus charm which chases off dementors with happiness. Harry finds it difficult to cast the Patronus charm and continues to hear his mother’s voice whenever a dementor approaches.
Sirius Black is spotted at Hogwarts twice over the course of the school year, but is never caught. Harry also begins to spot a large black dog periodically, which Trelawney believes is sign of Harry’s impending death. Midway through the term, Harry discovers that Sirius Black was James Potter’s best friend, Harry’s godfather, and the wizard responsible for betraying the Potters to to Lord Voldemort. Professor Lupin confirms this disturbing detail and admits to knowing both James Potter and Sirius Black during his youth.
As the end of term approaches, Harry learns that Hagrid’s hippogriff, Buckbeak, has been sentenced to death. Harry, Ron and Hermione decide to go to Hagrid’s hut to be with him during Buckbeak’s execution, even though it’s against the rules to venture off into the school grounds at night. Hagrid tries to save the trio from getting into trouble by pushing them out his back door as the executioner arrives. The large black dog appears and drags Ron into a secret tunnel beneath the Whomping Willow tree, which leads to an abandoned house called The Shrieking Shack in Hogsmeade. Harry and Hermione follow and discover a murderous Sirius Black waiting for them.
Black explains that he is an animagus with the ability to transform into a dog. He also reveals that he is not after Harry, but Ron’s rat scabbers who is also an animagus named Peter Pettigrew. The trio don’t believe him at first, but Lupin turns up in the Shrieking Shack and the two force Pettigrew to transform out of his rat shape. Lupin and Black tell Harry that it was Pettigrew, not Black, who betrayed Harry’s parents to Lord Voldemort. Lupin also confirms Hermione’s suspicion that he is a werewolf and that Harry’s father, Sirius Black, and Pettigrew learned to become animaguses to accompany Lupin during his monthly transformations. Although Lupin and Black want to kill Pettigrew to avenge James and Lily, Harry intervenes. They decide to take Pettigrew up to the school to clear Black’s name, but Pettigrew escapes when Lupin transforms into his wolf shape after exposure to the full moon. Dementors swarm Harry and Sirius and, as Harry faints, he sees someone who looks like his father cast a Patronus charm that drives the dementors away.
Harry awakens in the hospital wing with Hermione and an unconscious Ron. He learns that Black is being imprisoned in the school and about to be administered a “dementor’s kiss” which will remove his soul from his body. Dumbledore, who believes that Black is innocent, suggests that Harry and Hermione use Hermione’s time turner, which she secretly used to take an overloaded course schedule all year, to go back in time and save Sirius. Harry and Hermione turn the time turner and manage to save Buckbeak from execution without being seen. While waiting to save Sirius, Harry realizes that it was his patronus that saved his past self. He casts the charm which drives away the dementors and flies Buckbeak up to the tower where Black is being held prisoner. Black escapes with Buckbeak and goes into hiding. Harry returns to Privet Drive for the summer, but remains in correspondence with Sirius.
