Harry Potter faces another frustrating summer with the Dursleys. On his thirteenth birthday, an unfortunate encounter with his Uncle Vernon's sister, Marge, leads to a confrontation where she harshly insults Harry and his parents. Overcome with anger, Harry inadvertently causes her to inflate and float away, leaving him terrified of being expelled for using magic outside of school. With his belongings in tow, he hastily escapes.
Fortunately, Harry is soon found by the Knight Bus, a lifeline for stranded witches and wizards, which transports him to the Leaky Cauldron. There, Cornelius Fudge, the Minister for Magic, reassures Harry that he won't face punishment. Reuniting with his close friends, Ron Weasley and Hermione Granger, Harry learns of the dangerous implications surrounding his life: Sirius Black, a notorious criminal and an ally of Lord Voldemort, has broken out of Azkaban Prison and is believed to be after him.
The journey to Hogwarts takes a dark turn when the Hogwarts Express is invaded by Dementors, the terrifying guardians of Azkaban. One enters Harry's carriage, causing him to faint in terror, but fortunately, the new Defense Against the Dark Arts professor, Remus Lupin, is there to fend it off with a Patronus Charm. Upon arrival at Hogwarts, Headmaster Albus Dumbledore announces that Dorothy and her companions will patrol the school until Black is captured. Meanwhile, Rubeus Hagrid takes on the role of Care of Magical Creatures teacher, but his first lesson quickly descends into chaos when Draco Malfoy provokes a Hippogriff named Buckbeak, resulting in Buckbeak being sentenced to death.
Finding solace back in Gryffindor Tower, the students are alarmed to hear that the Fat Lady's portrait has been attacked, leaving her terrified. She informs Dumbledore that Black has gained entry into the castle, prompting a lockdown. During a thrilling Quidditch match, the Dementors interfere, causing Harry to tumble off his broomstick and land hard, destroying it against the Whomping Willow. Thankfully, Dumbledore employs the Slowing Charm to prevent a disastrous fall, and Harry eventually recovers in the infirmary.
After being discharged, Harry attempts to sneak into Hogsmeade but is caught by Fred and George, who gift him the Marauder's Map. With the help of this enchanted map, Harry ventures into Hogsmeade and overhears significant information regarding Sirius Black—discovering that he is actually Harry's godfather but was involved in revealing the Potters' location to Voldemort, leading to their demise, as well as the death of their friend Peter Pettigrew.
Determined to master the Patronus Charm to protect himself from the Dementors, Harry persuades Lupin to teach him. The plot thickens as Ron is dragged into a hidden passageway by a large black dog, leading the trio to discover that the dog is, in fact, Sirius Black, an Animagus. In a surprising turn of events, Lupin embraces Black, revealing their deep-rooted friendship while also admitting to his own werewolf condition.
As they piece together the past, the group reveals that Scabbers, Ron's pet rat, is in fact Pettigrew, who faked his death and framed Black. However, chaos ensues when the full moon forces Lupin to transform, leading to a harrowing battle where Black is heavily injured attempting to hold off his old friend. Amidst the turmoil, Harry finds himself in a battle against the Dementors, but just as hope seems lost, a powerful Patronus emerges, saving them before he loses consciousness.
Upon awakening in the infirmary beside Dumbledore and Hermione Granger, Harry learns that Sirius has been captured and faces the Dementor's Kiss. Acting on Dumbledore's advice, Harry and Hermione utilize her Time-Turner to go back in time, witnessing the night unfold before their eyes. They manage to rescue Buckbeak from execution and, in a climactic moment, Harry discovers that he was the very same figure who cast the Patronus before.
Together, they free Sirius, who escapes on Buckbeak, still a fugitive lacking proof of his innocence. They return to their timeline, where Lupin, acknowledging his condition, resigns from his teaching post and returns the Marauder's Map to Harry. To round off a tumultuous year, Harry receives a Firebolt broomstick from Sirius, igniting joy as he takes to the skies.
