Harry Potter begins his third year at Hogwarts, where he learns that Sirius Black, a dangerous prisoner, has escaped from Azkaban and is believed to be hunting him. The school is protected by Dementors, dark creatures that affect Harry deeply because of his past trauma.
As Harry investigates, he discovers hidden truths about his parents and Sirius Black. With Ron and Hermione’s help, he uncovers that Sirius is actually innocent and was falsely accused. The real traitor is Peter Pettigrew, who has been hiding in disguise.
Using a Time-Turner, Harry and Hermione go back in time to save Sirius and Buckbeak the Hippogriff. In the end, Sirius escapes, but remains a fugitive, and Harry gains a deeper understanding of his past and his parents’ friends.