Harry Potter and the Prisoner of Azkaban, starring Daniel Radcliffe, Emma Watson and Rupert Grint, finds Harry, Hermione and Ron in their third year at Hogwarts. Before school begins, Harry has to endure yet another summer with the Dursleys. This time, Aunt Marge comes to visit. Aunt Marge has never taken a liking to Harry, and her favorite amusement is picking on him. Eventually she goes too far, and Harry winds up inflating her. Figuring he is now expelled for using magic, he grabs his trunk, ready to start a life on the run.
Not too far on his way, a giant bus appears. It is the Knight Bus, ready to take wizards where they need to go. Harry decides on Diagon Alley.
When he arrives, he is greeted by the the Cornelius Fudge, the Minister of Magic. Instead of punishing Harry, they are happy to have found him safe. A dangerous criminal, Sirius Black, has escaped from Azkaban. Azkaban is the high security prison for wizards, which no one has escaped until now. Harry gets to spend the last two week of summer holiday exploring the shops and enjoying his time in the wizard world.
On the train to Hogwarts, Harry has his first encounter with dementors who guard Azkaban. Dementors are able to suck the joy out of the environment, leaving all in the area joyless. When the dementors enter Harry's train car, he hears the voice of his mother, trying to defend him from Voldemort when he was just a baby, and ends up passing out.
Malfoy learns of Harry's fear of dementors and tries to use it to his advantage on the quiditch field. Thanks to the instruction of the new defense against the dark arts teacher, Harry learns how to overcome the dementors.
Meanwhile, Hagrid is facing troubles of his own when he is appointed the new care of magical creatures teachers.
Hermione has a secret of her own. Using this secret, Harry and Hermione have a chance to save an innocent beast and man.