Harry Potter and the Prisoner of Azkaban is the third novel in the Harry Potter series.<p />
The third book opens with Harry Potter enduring another unhappy summer at the Dursleys'. One day, Harry overhears a news report about escaped convict, Sirius Black. When Uncle Vernon's sister, Marge, viciously insults Harry, his anger causes her to inflate and float to the ceiling. Upset, Harry runs away. On a dark street, he sees a large black dog ominously watching him from the bushes, but the Knight Bus suddenly appears and takes him to the Leaky Cauldron. During the trip, Harry learns that Black murdered thirteen people with one curse and is a supporter of Lord Voldemort. He is met by Minister for Magic, Cornelius Fudge, who Harry is certain will expel him from Hogwarts for using under-age magic. Surprisingly, the matter is dropped. While staying at the Leaky Cauldron, Harry hears Mr. and Mrs. Weasley arguing over whether he should be warned about Black.<p />
There are a few changes at Hogwarts as Harry begins his third year. For one, Hermione is taking nearly twice as many classes, including some taught at the same time. In addition, two new wizards join the staff: Professor Remus J. Lupin for Defence Against the Dark Arts and Rubeus Hagrid for Care of Magical Creatures. While Lupin's lessons are enjoyable, Hagrid's soon become dreary. During the first class, Draco Malfoy deliberately provokes the hippogriff Buckbeak, a half-horse, half-bird creature, into attacking him. Draco's father, Lucius Malfoy, files a complaint against Hagrid.<p />
Because Black is still at large, Dementors, the inhuman Azkaban prison guards, patrol the school. Dementors drain happiness from anything they approach. Harry is particularly affected, and Professor Lupin teaches him the Patronus charm that repels them after several Dementors approach Harry during a Quidditch match, causing him to faint and fall off his broomstick. Albus Dumbledore stops Harry's fall, but his Nimbus 2000 flies into the Whomping Willow and is destroyed.<p />
Meanwhile, tension grows between Hermione and Ron because Hermione's cat, Crookshanks, continually torments Ron's rat, Scabbers. At Christmas, Harry receives a superb Firebolt broomstick, although Hermione suspects Black is the anonymous donor. She reports it to Professor Minerva McGonagall, who confiscates the broom for testing. Harry and Ron are furious with Hermione and stop speaking to her. When the broom is returned some months later, the two boys try to make up with Hermione, but it goes wrong when Ron discovers Scabbers is missing; Ron blames Crookshanks.<p />
Shortly before Christmas, the Weasley twins give Harry their Marauder's Map, a magical document that shows every person's location within Hogwarts as well as secret passageways in and out of the castle. Harry uses a tunnel to sneak into Hogsmeade village where he overhears a disturbing conversation that Black was his parents' best friend and is his godfather and legal guardian. He was the Potters' Secret Keeper and he supposedly divulged the Potters' secret whereabouts to Lord Voldemort and murdered their friend Peter Pettigrew, as well as the twelve Muggle bystanders.<p />
After Harry completes his Divination Exam, Professor Trelawney enters a trance and predicts that the Dark Lord's servant will return to him that night. Harry and Ron finally make peace with Hermione, but the trio soon learn that Buckbeak will be executed. When they visit Hagrid to console him, Scabbers appears, bites Ron, and Ron chases him to the Whomping Willow. A large dog attacks Ron and drags him and Scabbers into a hole at the tree's base. Harry and Hermione follow, finding a tunnel leading to the Shrieking Shack. Inside, Harry confronts Sirius Black, who, as an unregistered, and therefore illegal, Animagus, can transform into an animal at will. Lupin, who spotted the group on the Marauder's Map, suddenly bursts in and embraces his old friend Black. Confronted by Hermione, Lupin admits to being a werewolf and also the Map's creator, along with Black, Pettigrew, and James Potter, the latter two also being illegal Animagi (a rat and a stag, respectively). Lupin and Black explain that Scabbers is actually Peter Pettigrew in his Animagus form. He is Voldemort's servant, and he betrayed the Potters, framing Black for the crimes. Harry is skeptical until Black and Lupin force Pettigrew back into his human form. Black explains he discovered that Pettigrew was still alive and escaped Azkaban to seek revenge. Harry stops Black and Lupin from murdering Pettigrew, believing that his father, James, wouldn't have wanted his two best friends to become killers.<p />
As the group heads back to the castle, the full moon rises, causing Lupin to turn into a werewolf. During the ensuing commotion, Pettigrew escapes. Black turns into his dog form to protect the others from Werewolf Lupin. Lupin flees, leaving Black badly injured. As Dementors move in to attack Black, Harry sees a mysterious figure in the distance cast a powerful stag-shaped Patronus, scattering the vicious creatures. Harry becomes convinced it is his father, or at least his father's spirit, who produced the Patronus. Black is then captured and taken to the castle where the Dementors intend to suck out his soul.<p />
Hermione reveals to Harry that she was entrusted with a time-traveling device, which is how she was able to attend so many classes. Prompted by Dumbledore, she and Harry travel three hours into the past, watching themselves go through the night's previous events. They set Buckbeak free and return to the Whomping Willow. As the dementors are about to attack the "other" Harry and Black, Harry realises that the mysterious figure he saw earlier was actually himself. He casts the powerful Patronus that repels the dementors. Harry and Hermione free Black, who escapes on Buckbeak as the timeline restores itself to normal.<p />