Harry, Ron and Hermione are excited to return to Hogwarts for their third year of wizarding education. However, their excitement is slightly dampened by the news that a dangerous criminal named Sirius Black has escaped from the wizarding prison, Azkaban, and is believed to be after Harry.
While on the Hogwarts Express, Harry and his friends encounter a group of vicious creatures called Dementors, who are searching for Black. Harry passes out and is saved by a new Defense Against the Dark Arts teacher, Professor Lupin.
Throughout the school year, Harry learns more about Black's connection to his family and the true story behind his parents' deaths. He also takes a keen interest in Lupin's lessons, which include practical applications of magic and combat against real-life monsters.
As the year progresses, Harry and his friends try to uncover the truth behind Black's motives and clear his name, while also contending with the growing danger posed by the Dementors. In the end, the truth is revealed and Harry's life is forever changed by the events of the year.