Harry (Daniel Radcliffe) and his friends Ron (Rupert Grint) and Hermione (Emma Watson) return for their third year at Hogwarts School of Witchcraft and Wizardry. Security is tightened because a dangerous wizard named Sirus Black (Gary Oldman) has esaped from Azkaban prison, and the prison guards, the frightening Dementors, are hunting him.
Harry and Ron find Hermione behaving peculiarly this year, popping up in classes at a moment's notice with no explanation. Meanwhile, Hagrid has been promoted from groundskeeper to Care of Magical Creatures teacher, but Harry's nemesis, Draco Malfoy (Tom Felton) causes a commotion which ends with Hagrid's pet Hippogriff, Buckbeak, being sentenced to death.
As Harry, Hermione and Ron race to save Buckbeak, Hermione's secret is revealed and Harry finds an unexpected ally.