Harry Potter and the Prisoner of Azkaban begins on Harry’s thirteenth birthday, when he receives gifts from his classmates via Owl Post. At breakfast the next morning, Harry notices on television that a man named Black has escaped from prison. Aunt Marge visits the Dursleys at this time, and she insults Harry’s parents several times. Inadvertently, Harry causes her to inflate. After a scary sighting of a big, black dog, Harry leaves the Dursleys’ residence and gets picked up by the Knight Bus. The Knight Bus lets Harry off in Diagon Alley, where the Minister of Magic, Cornelius Fudge, greets him. He hires a room and waits for school to begin. Harry finishes his coursework in Diagon Alley, admires a Firebolt broomstick in a shop window, and eventually runs into his friends Ron and Hermione. Hermione purchases Crookshanks, a cat who chases Scabbers, Ron’s aged pet rat, from a pet shop. Ron is irritated to no end. Harry overhears Ron’s parents discussing the fact that Sirius Black is after Harry the night before they all go for Hogwarts.
The kids board the Hogwarts Express train, but are briefly stopped by a Dementor. Professor Lupin, the new defence against the dark arts teacher, revives Harry after he passes out. The kids arrive at Hogwarts shortly after that, and classes begin. Professor Trelawney predicts Harry’s death in divination class by reading tealeaves and discovering a Grim, a giant black hound indicating death. Hagrid shows Hippogriffs to the students in his magical creature care lesson. Hippogriffs are huge, dignified hybrids between horses and eagles. Malfoy insults Buckbeak, one of the monsters, and is attacked. Malfoy prolongs the injury in the hopes of getting Hagrid dismissed and Buckbeak put to sleep. Professor Lupin leads the class in the defeat of a Boggart, which transforms into the viewer’s worst fear in Defense Against the Dark Arts. It transforms into an orb for Lupin and a spider for Ron. Harry has little hope of defeating it.
Harry enjoys tea with Professor Lupin during a Hogwarts tour to Hogsmeade, a wizard village Harry is unable to visit because to his lack of a permission form. Harry learns that the reason he wasn’t permitted to fight the Boggart was because Lupin was afraid it would take Voldemort’s form. Harry is taken aback by this concern, for he had been worrying even more about the terrifying Dementors. Snape brings Lupin a hot potion, which he drinks, much to Harry’s dismay. Sirius Black gets into Hogwarts later that night and destroys the Fat Lady picture guarding Gryffindor Tower. While the teachers search the castle, the pupils spend the night in the Great Hall. Soon later, Quidditch begins in earnest, with Gryffindor House taking against Hufflepuff. During the game, Harry notices a big black dog, followed by a swarm of Dementors. He falls from his broomstick and loses consciousness. When Harry wakes up, he discovers that his trusty broomstick has flown into the Whomping Willow and been destroyed in his fall, and that the game has been lost. Later, Harry learns from Lupin that the Dementors had such an impact on Harry due of his horrific history.
Fred and George Weasley give Harry the Marauder’s map, written by the mysterious quartet of Moony, Prongs, Wormtail, and Padfoot, on the next Hogsmeade visit, from which Harry is prohibited. This map leads Harry to Hogsmeade, where he meets up with Ron and Hermione through a secret entrance. Harry overhears Cornelius Fudge discussing Sirius Black’s involvement in the deaths of Harry’s parents and another Hogwarts student, Peter Pettigrew, who was blown to parts and left with only a finger, inside the Hogsmeade bar. Back at Hogwarts, Harry finds that Hagrid has gotten word that Buckbeak, the hippogriff who assaulted Malfoy, will be tried, and Hagrid is distraught. The winter vacations are approaching. Harry receives a Firebolt, the world’s most stunning racing broomstick, for Christmas. Hermione reports the broomstick to Professor McGonagall, who takes it away out of fear that it was sent (and cursed) by Sirius Black, much to his and Ron’s dismay.
After the holidays, Harry continues studying with Professor Lupin to use the Patronus charm to ward off Dementors; he is reasonably successful, but he is still unsure of his capacity to do so. Harry’s broomstick is given to him shortly before the Ravenclaw game, and as Ron carries it up to the dormitory, he discovers evidence that Scabbers has been eaten by Crookshanks. Hermione has enraged Ron. Gryffindor then plays Ravenclaw in Quidditch shortly after. On his Firebolt, Harry triumphs and wins the game. Sirius Black breaks into Harry’s room and tears the curtain around Ron’s bed after all the students have gone to bed. Hagrid brings Harry and Ron around for tea a few days later and chastises them for ignoring Hermione because of Scabbers and the Firebolt. They are slightly remorseful, but not in a bad way. Soon after, Harry joins Ron on a Hogsmeade trip while wearing his invisibility cloak; when he returns, Snape finds him and confiscates his Marauder’s Map. Lupin saves Harry from Snape’s rage, but not before severely reprimanding him for putting his life in danger for “a bag of magic tricks.” Harry runs into Hermione as he leaves Lupin’s office, and she informs him that Buckbeak’s execution date has been set. In their efforts to assist Hagrid, Ron, Hermione, and Harry have reconciled. Hermione is extremely upset by all of her work at this point, and in a single day, she slaps Malfoy for picking on Hagrid and quits Divination, reasoning that Professor Trelawney is a complete liar. Days later, Gryffindor defeats Slytherin in a shady Quidditch match to win the Cup.
Exams come around, and Professor Trelawney predicts the return of Voldemort’s servant before midnight during Harry’s meaningless Divination exam. Before the execution, Ron, Hermione, and Harry hide behind Harry’s invisibility cloak and go to comfort Hagrid. Scabbers is discovered in Hagrid’s milk jug by Hermione while at his cabin. Buckbeak is executed after they leave. As Ron, Harry, Harry, and Hermione are leaving Hagrid’s house, the enormous black dog approaches them, pounces on Ron, and pulls him under the Whomping Willow. Crookshanks, Harry, Hermione, and Crookshanks dash down after them; strangely, Crookshanks understands how to stop the flailing tree. They arrive at the Shrieking Shack after passing through an underground tunnel. They discover that the black dog has transformed into Sirius Black and has taken up residence in Ron’s room. Harry, Ron, and Hermione manage to disarm Black, but Professor Lupin enters the room and disarms him before Harry can kill Black and avenge his parents’ deaths. As Lupin and Black exchange a series of nods and embrace, Harry, Ron, and Hermione are stunned.
Lupin and Black explain everything after the three students have calmed down enough to listen. Lupin is a werewolf who is kept tame by a special steaming potion created by Snape for him. Animagi (people who can take on animal forms) were created by Lupin’s best friends, James Potter, Sirius Black, and Peter Pettigrew, while he was a student at Hogwarts, so that they may roam the grounds with him during full moon. They tell how, in a practical joke set up by Sirius, Snape pursued Lupin to his transformation location and was narrowly rescued by James Potter. Snape emerges from beneath Harry’s discarded invisibility cloak at this point, but Harry, Ron, and Hermione disarm him, knocking him out. The real murderer of Harry’s parents, according to Lupin and Black, is not Black, but Peter Pettigrew, who has been assumed dead but has been hiding all these years under the appearance of Scabbers. Scabbers is transformed into Pettigrew, who squeals and hedges before confessing and showing himself to be Voldemort’s servant and Black to be innocent. They all return to Hogwarts, but Lupin, who has neglected to take his regulating tonic (the steaming liquid), transforms into a werewolf when he sees the full moon. In order to protect Harry, Ron, and Hermione from Lupin, Sirius Black transforms into a big black dog. A swarm of Dementors confronts Black as he returns after driving the werewolf into the woods, paralysing him with terror. One of the Dementors is about to drain Harry’s soul because his patronus charm isn’t strong enough. A patronus appears from nowhere and drives the Dementors away. Harry passes out.
Snape and Cornelius Fudge are discussing the idea that Sirius Black is going to receive the fatal Dementor’s Kiss as Harry wakes up in the hospital wing. Harry and Hermione object, citing Black’s innocence, but to no use; then Dumbledore enters the room, shoos away the others, and inexplicably advises that Harry and Hermione journey back in time and save both Black and Buckbeak using Hermione’s time-turning device. Harry and Hermione are pushed into the past when Hermione turns her hour-glass necklace back three turns, and they save Buckbeak just before his execution. Harry observes the Dementor sequence from a hidden location in the woods and realises that he was the one who summoned the patronus, and he is both touched and perplexed to see that his patronus has taken the appearance of a deer, which he identifies as Prongs, his father’s animagi form. Harry and Hermione fly to the tower where Black is imprisoned after saving his former self from the Dementors, and they rescue Black, sending him away to freedom on Buckbeak’s back. Harry is disappointed to learn the next day that Professor Lupin is leaving Hogwarts as a result of the previous night’s fear. Dumbledore meets with Harry and offers him sound advise based on the events of the past. Harry receives an owl-post letter from Sirius on the train ride home, which includes a Hogsmeade permission letter, words of reassurance that he is safe in hiding with Buckbeak and that he was, in fact, the sender of the Firebolt, and a miniature pet owl for Ron. As he returns to the Dursleys for the summer, Harry feels slightly buoyed.