Harry Potter and the Prisoner of Azkaban
by J. K. Rowling
Harry is back at the Dursleys' for the summer holidays, where he sees on Muggle television that a convict named Sirius Black has escaped from prison. After the Dursley's Aunt Marge personally insults Harry, Harry accidentally inflates her, then runs away from home, fearing expulsion from school. After being picked up by the Knight Bus, meeting Stan Shunpike, and encountering a large black dog that seems to be watching him, he travels to the Leaky Cauldron, where Cornelius Fudge, the Minister for Magic, asks Harry to stay in Diagon Alley for his own protection. While there, he reunites with his best friends Ron Weasley and Hermione Granger.
Before leaving for Hogwarts, Harry learns from Arthur Weasley that Sirius Black is a convicted murderer from the wizarding world and that Black has escaped from the wizard prison Azkaban to kill Harry. On the way to Hogwarts, a Dementor boards the train, causing Harry to relive his parents' deaths before fainting. The new Defence Against the Dark Arts teacher, Remus Lupin, protects Harry, Ron, and Hermione from the Dementor. They learn Dementors will be patrolling the school in an attempt to catch Black. Although Professor Lupin is popular with his students, the Potions master, Snape, seems to hate him.
While Third Years are allowed to visit the all-wizarding village of Hogsmeade on holiday, Harry is blocked from going because he has no permission slip from his legal guardian. Fred and George Weasley give him the Marauder's Map, which is enchanted to show all passages and people on Hogwarts grounds, to sneak out. Lupin later catches Harry with the map and confiscates it, believing it endangers Harry.
During a Quidditch match, Harry faints and falls off his broomstick after Dementors invade the field. His Nimbus 2000 is blown away and smashed by the Whomping Willow, a violent tree in the grounds. Afterward, Professor Lupin teaches Harry how to defend himself from Dementors using the Patronus charm.
Ron and Hermione's friendship suffers when Ron believes Hermione's cat, Crookshanks, to have eaten his rat, Scabbers. Hermione also gets Harry's new anonymously given Christmas present, a top-of-the-range Firebolt broomstick, confiscated for fear it might be jinxed by Sirius Black. Hermione becomes stressed by her many classes, some of which seem to occur at the same time, a detail which she refuses to explain to Harry and Ron.
Meanwhile, Hagrid's hippogriff Buckbeak is in danger of being executed by the Ministry of Magic after injuring Draco Malfoy, who provoked Buckbeak. Despite Hermione's and Ron's efforts to defend the hippogriff, they and Harry seemingly hear Buckbeak executed as they leave Hagrid's hut. While visiting Hagrid's, however, Hermione discovers Scabbers hiding there.
Scabbers escapes from Ron, who gives chase. The large black dog appears and attacks Ron, then drags him into a tunnel under the Whomping Willow. Crookshanks (who seems to be in league with the dog), Harry, and Hermione follow them through the tunnel and to the Shrieking Shack, a haunted house in Hogsmeade. They discover that the dog is Black in animal form. Harry disarms Black and intends to kill him but is unable to do so.
Lupin enters and embraces Black as a friend. He explains that, although he believed Black to have betrayed the Potters, he realises now that it was Peter Pettigrew. He admits he is a werewolf and that, while at school, the Marauders (Black, Pettigrew, and Harry's father, James Potter) learned to transform into animals to tame Lupin during his transformations. Lupin explains that Scabbers is Pettigrew in his animal form. Pettigrew faked his own death, framing Black for the murders, and has been hiding from Black since his escape from Azkaban.
Snape, who holds a childhood grudge against Lupin and his friends, arrives to apprehend Black but is knocked unconscious by Harry, Ron, and Hermione, who now believe Black to be innocent. Lupin and Black transform Pettigrew into human form and prepare to kill him, but Harry stops them, feeling his father would not have wanted it. He convinces Lupin and Black to send Pettigrew to Azkaban instead.
As they attempt to bring Pettigrew back to Hogwarts, the full moon transforms Lupin into a wolf. The now feral Lupin attacks them, and in the confusion, Pettigrew escapes. Harry, Hermione, and Black are chased to the shore of the lake, where they are set upon by Dementors. They are rescued by a Patronus cast by a figure on the opposite shore, whom Harry thinks is his father.
Reawakening in the hospital wing, Harry and Hermione are told that Black has been sentenced to have his soul sucked out by Dementors, a fate worse than death. Professor Dumbledore tells Harry and Hermione to use Hermione's time-turner, a device she has been using to attend her simultaneous classes, to go back in time and save Buckbeak and Sirius. They rescue Buckbeak from execution and watch themselves attacked by Dementors at the lake. Harry realises that the figure he saw was not his father but himself, and casts the Patronus that drives away the Dementors. Harry and Hermione ride Buckbeak to the tower where Black is held and break him out. Black escapes on Buckbeak.
Harry and Hermione return to the hospital wing to close the timeline. Enraged by the escape of Black, Snape lets slip that Lupin is a werewolf, forcing him to resign. Harry returns to Kings Cross with his friends and receives a letter from Black, giving him permission to visit Hogsmeade next year.