Harry Potter and the Prisoner of Azkaban is the third novel in J.K. Rowling's Harry Potter series. Harry learns that a prisoner known only as Black has escaped from Azkaban, a high security prison. Harry learns that "Black" was once a friend of his parents, though he was rumored to have betrayed them and gave Voldemort access to their house, where he killed Harry's parents. Harry must discover the true identity of Black and determine what role he will play in the ever approaching battle.
Harry is back at the Dursleys, where he sees on Muggle television that a prisoner named Sirius Black has escaped. Harry involuntarily inflates Aunt Marge when she comes to visit after she insults Harry and his parents. This leads to his running away and getting picked up by the Knight Bus. He travels to Diagon Alley, where he meets Cornelius Fudge, the Minister for Magic, who asks Harry to stay in Diagon Alley for the remaining three weeks before the start of the school year at Hogwarts.
The night before he is expected to leave for Hogwarts, Harry learns that the escaped prisoner, Sirius Black, is a convicted murderer, and it is believed that he wants to murder Harry. On the way to Hogwarts, a Dementor boards the train, causing Harry to faint, but he is helped by the new Defence Against the Dark Arts teacher, Remus Lupin. Harry, Ron, and Hermione learn that the Dementors will be patrolling the school in an attempt to protect Harry from Black.
At Hogwarts, Harry has several problems with the Dementors, including an episode at a Quidditch match, during which he faints and falls off his broomstick from around 50 feet in the air. His broom is smashed by the Whomping Willow. Working with Harry outside class, Lupin teaches him the Patronus Charm to repel Dementors.
On an unauthorized visit to the village of Hogsmeade (thanks to the The Marauder's Map, given to him by George Weasley and Fred Weasley), Harry overhears some of his teachers talking with Fudge about Black, saying that Black was a friend of Harry's parents, but he betrayed them and gave Voldemort access to their house, and that he also killed thirteen Muggles and his former friend Peter Pettigrew.
Ron and Hermione's friendship later suffers when Ron believes that Hermione's cat, Crookshanks, ate his rat, Scabbers. At Christmas, Harry receives a mysterious present, a late-model Firebolt broom. Fearing it might be cursed, Hermione reports the gift to Professor McGonagall, which leads to bad feelings between her and Ron and Harry.
Ron, Hermione, and Harry join the effort to save Hagrid's hippogriff, Buckbeak, from being executed for attacking Draco Malfoy, after Draco provoked him. Their efforts are unsuccessful, but Scabbers reappears shortly after they hear Buckbeak being executed.
Ron chases Scabbers, only to be attacked by a big black dog, which Harry has seen several times before. The dog drags Ron through a tunnel under the Whomping Willow into the Shrieking Shack. Harry and Hermione follow, and there is a brief standoff when they find Ron and Sirius, who has transformed from the dog. Lupin enters, and they explain the situation to Harry and his friends: Lupin is a werewolf, which led to his friends James Potter, Sirius Black, and Peter Pettigrew becoming animagi. Lupin explains that Scabbers is Pettigrew in his animal form; he has been hiding from Black, whom he had framed for the murders of Harry's parents and the thirteen Muggles.
Professor Snape arrives to apprehend Black but Harry knocks him unconscious. Lupin and Black transform Pettigrew back into human form and prepare to kill him, but they are stopped by Harry, as he feels his father would not have wanted it.
As they move back toward Hogwarts, Lupin turns into a werewolf and becomes violent, having missed a dose of a potion he had been taking that would allow him to keep his human mind when in animal form. Pettigrew escapes again, but Black prevents Lupin, in his werewolf form, from attacking the others. Some Dementors approach, and the three lose consciousness.
When they wake up in the hospital, Harry, Ron, and Hermione are told that Black has been sentenced to receive the Dementor's kiss, which removes the soul of the recipient. Dumbledore advises Hermione and Harry to use Hermione's time-turner, a device she has been using to double-up on classes, which permits them to go back in time and save Buckbeak, who carries Black away to safety.