Having further penetrated the mysteries of Hogwarts Academy, where Voldemort had once been a pupil under the name of Tom Riddle in Harry Potter and the Chamber of Secrets, Harry faces sterner challenges in his third year at the school. The infamous Sirius Black has escaped from Azkaban—the magical prison to which Voldemort’s key supporters, the Death-Eaters, were sent when his first campaign was thwarted—and is rumored to be making his way to the school in order to kill Harry.
Harry begins to realize that the matter is more complicated when he has a terrifying encounter with a Dementor—one of the hideous, soul-sucking entities which guard the prison—on the Hogwarts Express. Even Dumbledore seems to doubt his testimony in this regard but is determined to keep him safe, with the aid of Remus Lupin, the new teacher of Defense Against the Dark Arts (a post with a remarkably high turnover). Harry has further trouble with both Severus Snape and Draco Malfoy, and his situation is further complicated when Hagrid becomes distraught after one of his unruly protégés, the hippogriff Buckbeak, is condemned to be put down.
It transpires that Sirius Black is actually Harry’s nearest living relative and staunchest defender, having once been part of a group of friends with Harry’s father and Remus Lupin, whose affliction with lycanthropy the group had striven to protect. The true enemy and cause of Harry’s troubles is, as always, Voldemort, who has subverted Azkaban and now has the Dementors at his beck and call. Harry is able to call upon the aid of the Marauder’s Map, which allows the locations of the academy’s inhabitants to be determined, as well as a time-bending watch, but he, Hermione, and Ron are tested to the limit of their strength and ingenuity by the quest to save Sirius and Buckbeak and to thwart Voldemort’s plans for a third time.