Harry Potter and the Prisoner of Azkaban recap

In his third year, Harry learns that Sirius Black, a convicted murderer said to have served Voldemort, has escaped Azkaban and is thought to be hunting him. Soul-draining prison guards called Dementors are posted around Hogwarts and affect Harry badly, so his new Defence teacher, Remus Lupin, teaches him to fight them off with the Patronus charm. Harry discovers that Black is his godfather and was blamed for betraying his parents to Voldemort. When Black is finally cornered, the truth comes out: he is innocent. The real traitor is Peter Pettigrew, who faked his own death and has spent years hiding as Ron's pet rat. Before Pettigrew can be handed to the authorities he escapes, so Black remains a wanted fugitive. Using a time-travel device, Harry and Hermione save both Black and a condemned creature, and Black flees to freedom. Harry ends the year knowing the truth about his parents' betrayal and having gained, then lost, the chance of a home with his godfather.

Harry Potter, orphaned as a baby when the dark wizard Lord Voldemort killed his parents and was defeated trying to kill him, was raised by his unkind aunt and uncle before learning at eleven that he is a wizard. At Hogwarts he was sorted into Gryffindor and became close friends with loyal Ron Weasley and clever Hermione Granger. In his first year he foiled Voldemort's attempt to steal the immortality-granting Philosopher's Stone. In his second, the hidden Chamber of Secrets was reopened and a monster began Petrifying students; Harry killed the serpent and destroyed an enchanted diary that held a memory of the teenage Voldemort, saving Ron's sister Ginny. Voldemort remains alive but powerless and in hiding, and the headmaster, Dumbledore, has warned that he means one day to return to full strength.

After losing his temper and accidentally inflating his visiting aunt Marge, Harry flees the Dursleys, expecting expulsion, and is instead collected by the wizarding authorities and given a room at a pub. There he learns that a notorious prisoner, Sirius Black, has escaped Azkaban, the wizard prison, and that Black was a supporter of Voldemort who murdered thirteen people; overheard talk suggests he is coming after Harry. On the train back to school, the ride is stopped by Dementors, the grim, soul-draining creatures that guard Azkaban and are now searching for Black. One of them affects Harry so badly that he faints, and the new Defence Against the Dark Arts teacher, the kind and shabby Remus Lupin, drives it off and revives him.

The school year gets under way with Dementors stationed at the castle's edges as a constant, oppressive presence, and Harry, who feels their effect worse than anyone, privately asks Lupin to teach him to defend against them. Hagrid, now the Care of Magical Creatures teacher, introduces a proud winged creature called a hippogriff, which injures Draco Malfoy after Draco insults it, and the Malfoys begin pressing for both Hagrid and the creature to be punished. Harry, kept off broomsticks and closely watched, chafes at being shielded. Lupin quietly becomes his favourite teacher. The dread around the castle sharpens on the night Sirius Black breaks into Hogwarts itself, slashing his way into the Gryffindor common room while the students are elsewhere, though he vanishes before he can be caught.

Using a magical map of the castle given to him by Ron's mischievous twin brothers, Harry sneaks into the nearby village of Hogsmeade, where he overhears the shattering claim that Sirius Black is his godfather, that Black was entrusted with keeping Harry's parents' hiding place secret, and that Black betrayed them to Voldemort and then murdered a wizard named Peter Pettigrew and a street full of onlookers. Consumed by hatred, Harry begins learning the Patronus charm from Lupin, slowly mastering a defence against the Dementors. The hippogriff's appeal fails and it is sentenced to death, deepening the friends' distress. The strain between Ron and Hermione worsens when Ron's rat, Scabbers, disappears and Ron blames Hermione's cat.

On the day the hippogriff is due to be executed, the three friends visit Hagrid to comfort him and find Scabbers alive after all. As they leave, an enormous black dog drags Ron and the rat into a tunnel beneath the violent Whomping Willow, and it leads to a derelict, haunted house. There the dog turns into Sirius Black. Lupin arrives, and together the two old friends reveal the truth Harry has never known: Black is innocent. The man everyone believed dead, Peter Pettigrew, faked his own murder, framed Black, and has been hiding for twelve years disguised as Ron's pet rat. Lupin, it emerges, is a werewolf and was a schoolfriend of Black and Harry's father. They force Pettigrew back into human shape, and he confesses that it was he who gave Harry's parents to Voldemort.

Harry stops Black and Lupin from killing Pettigrew, choosing to have him handed to the authorities so that Black can be cleared. But as they cross the grounds the full moon rises, Lupin transforms into an uncontrolled werewolf, and in the chaos Pettigrew escapes as a rat. Dementors close in and nearly destroy Harry and Black, until Harry casts a fully formed Patronus that drives the whole swarm back, realizing he could do it because he had already seen himself do it. Black is recaptured and sentenced to the Dementor's Kiss, which destroys the soul. Following a hint from Dumbledore, Hermione uses a Time-Turner she has secretly carried all year to take herself and Harry back a few hours; they free the condemned hippogriff and use it to rescue Black, who flies away to freedom. Because Pettigrew is gone, Black cannot be proven innocent and must remain a fugitive. Lupin resigns after word of his condition spreads, and Harry returns to the Dursleys knowing the truth about his parents at last, and mourning the home he was briefly offered.

Cornered in a haunted house, Sirius Black is revealed to be innocent: he never betrayed Harry's parents and never killed anyone. The true traitor is Peter Pettigrew, who faked his death twelve years ago, framed Black, and has been hiding ever since as Ron's pet rat, Scabbers. Lupin, revealed to be a werewolf and Black's old friend, helps expose him. The truth clears Black, but on the way back to the castle Lupin transforms under the full moon, Pettigrew escapes in the confusion, and Dementors swarm Harry and Black. Harry drives them off with a powerful Patronus, having earlier glimpsed himself doing it. Black is recaptured and sentenced to have his soul destroyed, but Hermione's time-travel device lets her and Harry go back a few hours to free the condemned creature Buckbeak and rescue Black, who escapes on its back. Because Pettigrew got away, Black cannot be cleared and stays a hunted fugitive, so the home Harry was briefly offered slips out of reach. Lupin resigns once his condition becomes known. Voldemort's servant Pettigrew is now free to seek his master, and Harry ends the year knowing at last the truth about how his parents died and who really betrayed them.