Harry Potter and the Prisoner of Azkaban
Harry's thirteenth birthday starts off as his best yet, with presents from his friends sent by owl. But life with the Dursleys is never pleasant for long, and when Uncle Vernon's sister Aunt Marge comes to visit, it gets decidedly less so. After Marge insults Harry and his parents, Harry inflates her like a balloon and takes off into the night before the Dursleys can stop him. Just after he sees a scary black dog, the magical Knight Bus picks him up. On board he learns that an escaped prisoner who had appeared on the television news is really the wizard Sirius Black, who once killed twelve Muggles and a wizard with one powerful curse.
Harry travels to Diagon Alley, where he spends the last two weeks of his vacation. Eventually, Ron and Hermione join him. Hermione buys a cat named Crookshanks that constantly tries to attack Ron's pet rat, Scabbers. Before they leave for Hogwarts, Harry overhears Ron's parents, Mr. and Mrs. Weasley, say that Sirius Black is after him.
On their way to Hogwarts, the train is stopped by a dementor, whose presence causes Harry to faint. Dementors are the soulless guards of Azkaban, now set to protect Hogwarts (and Harry) against an attack from Sirius Black. The new Defense Against the Dark Arts teacher, Professor Remus Lupin, revives Harry. The other new teacher, Harry is pleased to learn at the welcome banquet, is Hagrid, who will teach the Care of Magical Creatures course. Classes begin with an actual bad omen when Harry's Divination teacher, Professor Sibyll Trelawney, tells him she sees a Grim in his future — a dark dog that portends death. Things go from bad to worse when one of the hippogriffs Hagrid introduces in class, Buckbeak, attacks Draco Malfoy, who later uses his father's influence to demand Buckbeak be put to death. Harry, Ron, and Hermione help Hagrid defend him.
Harry hasn't gotten permission from his guardians (the Dursleys) to visit Hogsmeade, a nearby wizarding village, so he must stay behind when his classmates take a trip there. Instead he spends time with the kind Professor Lupin. Disturbing events continue when the students return to find the portrait of the Fat Lady that guards the entrance to Gryffindor Tower shredded and slashed. Peeves the Poltergeist says that Sirius Black is the culprit. During the next Quidditch match, Harry sees another black dog, as well as dementors, who cause Harry to faint once again. He falls off his broomstick, which is destroyed by the Whomping Willow.
Things get a little brighter when Fred and George Weasley present Harry with the Marauder's Map, allowing him to sneak out of Hogwarts castle to Hogsmeade. There he learns that Sirius Black had once been Harry's father James's best friend, and James and Lily Potter even named Sirius Harry's godfather! But Sirius was also James's greatest betrayer, as he told Voldemort of the Potters' whereabouts after the family went into hiding.
For Christmas, Harry receives a top-of-the-line Firebolt broomstick from an unknown sender, but Professor McGonagall takes it from him, fearing it is a cursed gift from Sirius Black. Luckily, it is found to be safe before the next Quidditch match, which Harry wins, as well as the final match against Slytherin, where Gryffindor captures the Quidditch Cup. Meanwhile, Lupin teaches Harry the Patronus Charm to protect him against dementors, and Gryffindor Tower suffers another attack from Sirius Black. Soon after, Harry gets caught visiting Hogsmeade and Lupin confiscates the Marauder's Map.
Exams begin, and during Harry's Divination test, Professor Trelawney suddenly predicts that a servant of Lord Voldemort will set out to rejoin his master before midnight. Despite Harry, Hermione, and Ron's best efforts, Buckbeak has been sentenced to death, and the trio go to Hagrid's cabin to comfort him. Immediately after the execution is carried out, Crookshanks takes off after Scabbers, and when Ron chases after them, he is attacked by a jet-black dog who drags him under the roots of the Whomping Willow. Crookshanks leads Harry and Hermione into a tunnel beneath the Willow, and they come out in the Shrieking Shack. There they learn the dog is really Sirius Black, who's an Animagus — a wizard who can take animal form. Harry and Sirius fight, and just as Harry is poised to kill his parents' betrayer, he finds himself unable to do it. Lupin arrives and confusingly embraces Sirius; Hermione reveals that Lupin is a werewolf, and Lupin tells them all that Scabbers too is an Animagus — that he's really, in fact, a wizard named Peter Pettigrew.
Lupin tries to explain it all. James Potter, Sirius Black, Peter Pettigrew and Lupin were best friends at Hogwarts, and the first three became Animagi so they could keep Lupin company when he took his wolf form once a month. But before Lupin can finish his story, Professor Snape appears and cuts the tale short, as he harbors a grudge against Black and relishes turning him in. But Harry, Ron, and Hermione disarm and Stun Snape, and soon Scabbers is indeed revealed to be the traitorous Pettigrew — the one who truly betrayed the Potters to Lord Voldemort. The whole group leaves the Shrieking Shack to hand Pettigrew over to the authorities. Sirius offers to let Harry come live with him once his name is cleared, an offer Harry eagerly accepts; but as they emerge into the full-moon night, Lupin transforms into a werewolf. A struggle ensues, allowing Pettigrew to escape and the dementors to arrive. Just before Harry is to receive the Dementor's Kiss, which would suck out his soul, a Patronus shoots out from nowhere to save him — but who sent it? Harry passes out on the grass by the lake.
When he regains consciousness, Harry finds himself in the hospital wing at Hogwarts with Ron and Hermione. He learns that Sirius has been recaptured and no one but Dumbledore believes he's innocent. Dumbledore tells Hermione to use her Time-Turner, a device that allows her to repeat hours, to save "more than one innocent life." Harry and Hermione go back in time to save Buckbeak from execution, and Harry realizes that he himself conjured the Patronus that rescued him from the dementors. He and Hermione fly Buckbeak to the tower where Sirius is imprisoned and set him free; he disappears with the hippogriff into the night. The book ends with a letter from Sirius, enclosing a permission slip for Harry to visit Hogsmeade.