Harry (Daniel Radcliffe) is in Purgatory, living for the summer with the Dursleys. Aunt Marge (Pam Ferris) insults his mother, and he makes her blow up like a balloon and float away. Then he storms out of the house with his broom and a trunk. Every time he goes to Hogwarts, it seems, he takes a different magical way. This time, he takes the Knight Bus, which is a mad sight to behold. At the Leaky Cauldron, he is pardoned by the Minister for Magic, Cornelius Fudge (Robert Hardy) for unauthorised use of magic. He reunites with his friends Ron (Rupert Grint) and Hermione (Emma Watson) and learns that Sirius Black (Gary Oldham) has escaped from Azkaban Prison and wants to kill him.

They continue on to Hogwarts on the Hogwarts Express, which is boarded by ghostly prison guards called Dementors, searching for Sirius Black. One enters Harry’s compartment, and he passes out, but new Defence Against the Dark Arts teacher (they go through those pretty quickly) Remus Lupin (David Thewlis) repels the creature with a Patronus charm. At Hogwarts, Headmaster Albus Dumbledore (Michael Gambon replacing the recently deceased Richard Harris) announces that the Dementors will be patrolling the school until Sirius Black has been captured. Hagrid (Robbie Coltrane) is the new Care of Magical Creatures teacher. His first class goes awry when bully Draco Malfoy (Tom Felton) teases the hippogriff Buckbeak into attacking him. His father Lucius Malfoy (Jason Isaacs) has Buckbeak sentenced to death.

Sirius Black is reported to have entered the castle. During a Quidditch match in a storm, Dementors attack Harry, causing him to tumble from his broom and the broom to fly into the Whomping Willow. During an outing at Hogsmeade, Harry learns that Sirius Black had been his father’s best friend, who apparently betrayed Harry’s parents to Voldemort, though he is Harry’s godfather. Remus Lupin teaches Harry the Patronus charm. After they witness the apparent execution of Buckbeak, Ron’s pet rat Scabbers bites him and escapes. A large dog appears and drags Ron and Scabbers into a hole beneath the Whomping Willow.

Harry, Ron, and Hermione follow an underground passage into the Shrieking Shack, where they find out the dog is actually Sirius (Gary Oldman), a canine animus. Lupin arrives and embraces Sirius. Lupin is a werewolf and explains that Sirius is innocent, falsely accused of betraying Harry’s parents. But they were betrayed by their so-called friend Peter Pettigrew (Timothy Spall), who has taken on the shape of Scabbers the Rat. Severus Snape (Alan Rickman) arrives to arrest Sirius, but Harry knocks him out with an Expelliarmus Spell. Lupin and Sirius force Pettigrew back into human shape. Harry convinces them not to kill him, but to turn him over to the Dementors. The full moon rises, and Lupin transforms into a werewolf. Sirius transforms into a dog and protects the others. Pettigrew becomes a rat again and escapes. Harry and Sirius are attacked by Dementors, and Harry sees a glowing stag in the distance, who saves them by casting a Patronus Spell. Harry passes out, but he believes the stag was his father.

Sirius is captured and sentenced to the Dementor’s Kiss. Dumbledore encourages Harry and Hermione to travel back in time and watch themselves and Ron in the past. They save Buckbeak from execution and witness the Dementors overpowering Harry and Sirius. It is Harry himself who conjured up the Patronus Spell. They rescue Sirius, who escapes with Buckbeak. As a werewolf, Lupus has to resign from teaching. He gives Harry the secret Marauder’s Map to the school, and Sirius sends him a new Firebolt broom.

The film was directed by Alfonso Cuaron, written by Steve Kloves, and produced by Chris Columbus, David Herman, and Mark Radcliffe (not related to Daniel). The movie took a left-hand turn into darkness, with shape-changers, betrayal, and the Dementors, who are truly terrifying. It is widely considered to be the best of the Harry Potter films, with some of the greatest actors in the U.K. portraying some of the most wonderful characters in popular literature.

When Cuaron was hired, he had not read any of the books or seen the movies. His friend Guillermo del Toro told him not to be stupid, read all the books immediately. Cuaron had a clause in his contract forbidding him to swear in front of the kids. Daniel Radcliffe had to appear awed in a scene and Cuaron told him to pretend he was seeing Cameron Diaz in a G-string. It was Cuaron’s idea to have the Hogwarts choir sing the “Double, double, toil and trouble,” lines spoken by the weird sisters in Macbeth. There is a wizard band in the novels called The Weird Sisters, and a real Canadian band named the Wyrd Sisters tried to sue. Cuaron asked the three leads to write an essay about their characters. Daniel Radcliffe wrote a simple one-page summary, Rupert Grint never turned his in, and Emma Watson wrote a sixteen-page essay. How’s that for casting?

Gary Oldman gave Daniel Radcliffe a bass guitar. Ian McKellen turned down the role of Dumbledore because he had appeared as Gandalf. He said he had enough trouble living up to one legend. Two would be too much. Emma Watson, supposed to slap Tom Felton, actually smacked the daylights out of him and didn’t know why. The bats flying around Hagrid’s hut are real. Unfortunately, they urinated everywhere. It seems that most of the actors took the job to impress their children. Crookshanks the Cat was played by Crackerjack and Pumpkin. Their shed fur was glued to their bodies to give them a mangy appearance.

All of the clothing was re-designed by costume designer Jany Temime, except for Severus Snape, who never appeared in anything but his billowing black robes. Buckbeak the Hippogriff is seen pooping. It was claimed as the first time a CGI animal did that, but it happened in Walking with Dinosaurs in 1999. Considered before Michael Gambone replaced Richard Harris as Dumbledore were Christopher Lee, Ian McKellen, Peter O’Toole, and Sir Richard Attenborough. In the end credits, footprints walk through the Marauder’s Map. John William’s feet are seen dancing. Professor Trelawny’s magnifying glass spectacles made Emma Thompson dizzy. It took six months to create the Dementors, which were imagined by J.K. Rowling in a fit of depression.