Harry Potter is, as usual, spending his summer at the Dursley's house. Uncle Vernon's sister Marge arrives for a visit and at dinner time viciously insults Harry and his parents. Harry's anger and unconscious magical powers cause her to inflate and float away. Expecting to have been expelled from Hogwarts for using magic outside of school, Harry flees. Suddenly, the Knight Bus appears and takes Harry to the Leaky Cauldron. There, Minister for Magic Cornelius Fudge tells Harry he will not be expelled. Harry learns that Sirius Black, an alleged Lord Voldemort supporter, has escaped from Azkaban, and is informed by Arthur Weasley that Black will likely come after him.

Harry, along with his best friends Ron and Hermione, journey to Hogwarts where they are now entering their third year. On the train they share a compartment with the sleeping, new Defense Against the Dark Arts teacher, Remus Lupin. Abruptly, the Hogwarts Express is stopped and a Dementor boards, searching for Black. The creature has a profound effect on Harry, causing him to faint. Lupin awakens and repels the Dementor. At school, Albus Dumbledore informs the Hogwarts students that because Black is at large, Dementors have been set up around the school as a protection. In addition to Lupin's appointment, Hagrid is announced as the new Care of Magical Creatures teacher. As lessons begin, Hermione frequently appears out of nowhere upon arrival to their various classes. Lupin's lessons prove enjoyable, with him teaching useful spells and helping less confident students, such as Neville. However Hagrid's do not go to plan, as during his first class, Draco Malfoy deliberately provokes the Hippogriff Buckbeak into attacking him. His father successfully has Buckbeak sentenced to death.