Sirius Black, the notorious murderer, has somehow evaded the Dementors (vicious, spirit-type creatures) and escaped from the impenetrable Azkaban Prison. Mr Weasley, from the Ministry of Magic, warns Harry that he is in serious danger as Sirius is headed for Hogwarts to find him. It’s believed that it was Sirius Black (Harry’s Godfather) who betrayed Harry’s parents to Voldemort. This led to their death.
While the students are on their way to Hogwarts on the train, it suddenly comes to a stop and all the lights go out. The window panes freeze over and a Dementor enters Harry’s carriage and tries to drag him away, causing him to faint. The Dementor might have succeeded but for Professor Lupin, teacher of Defence of the Dark Arts, who overcomes the creature with his wand. At Hogwarts, Dumbledore explains that Dementors are guarding the grounds to prevent Sirius Black from entering. He warns the students to keep well away.
Dumbledore and the teachers try to keep the school operating normally and Lupin starts to teach the young wizards and witches about boggarts. Boggarts are figments of the imagination that appear to students in the form of their most feared object. To combat them, students learn to yell out ‘ridiculous’ and the boggart changes into something funny. The Divination teacher Professor Trelawney fears for Harry and predicts disastrous events for him. Hagrid, the giant groundskeeper, tries to teach the students how to befriend a hippogriff called Buckbeak. Harry succeeds but when Malfoy (a particularly nasty student whose father has some influence in running the school) tries, Buckbeak rears up and kicks him in the shoulder. Malfoy immediately seeks the dismissal of Hagrid but has to be satisfied with the execution of Buckbeak.
Harry and his two friends, Ron and Hermione, try to console Hagrid but are very upset themselves. On their way back from Hagrid’s house, a wolf runs at them. It grabs Ron by the ankle and dragging him into a hole beneath the Whomping Willow tree. Harry and Hermione try to enter the hole but the tree does its best to keep them out, beating them back with its branches and sending Hermione flying. They eventually enter and find Ron with a broken leg, unable to move. He tries to warn them that it’s a trap, but he is too late. Sirius is there waiting for them. Harry runs at Sirius and nearly chokes him but is prevented by the arrival of Lupin.
Sirius explains to Harry that it was in fact Peter Pettigrew who betrayed Harry’s parents, not Sirius. Some had thought that Peter Pettigrew had also been killed with Harry’s parents but he has actually been living as Ron’s pet rat Scabbers for the past 12 years. Sirius makes Scabbers turn back into Peter and confess his crimes.
At midnight, things start to turn nasty when Lupin turns into a werewolf, Peter turns back into a rat and runs away, and Sirius turns into a wolf. Sirius and Lupin fight ferociously and when Sirius becomes human again he has an injured shoulder and is unable to fight off the Dementors who are waiting for him. Harry, however, who has been taking private lessons from Lupin on how to defend himself against the Dementors, manages to overpower them, thinking that it is in fact his father casting the spell. Harry is overcome by his efforts and Sirius appears dead. They are both taken back to Hogwarts, where Sirius is locked in the tower awaiting return to Azkaban.
Dumbledore saves the situation with a plan in which he engages Hermione’s help, hoping to save the life of more than one innocent victim.