The Escape of Sirius Black: The book opens with the news that Sirius Black, a dangerous criminal and Harry's godfather, has escaped from Azkaban prison. Black is rumored to be after Harry, adding a new level of fear to the already-stressful environment.
New Teachers and a Mysterious Prisoner: Two new teachers arrive at Hogwarts: Remus Lupin, the Defense Against the Dark Arts teacher, and Professor Trelawney, the Divination teacher. Meanwhile, a mysterious prisoner, presumed to be Sirius Black, is held in the castle.
The Patronus Charm: Harry learns the powerful Patronus Charm from Remus Lupin, which helps him fight off the Dementors, guards of Azkaban who are drawn to his sadness and fear.
The Truth About Sirius Black: Through various clues and encounters, Harry discovers the truth about Sirius Black. He was falsely accused of betraying Harry's parents to Lord Voldemort, and his real mission was to protect Harry.
Facing the Truth: Harry must confront the truth about his parents' death and the real motives of Sirius Black, as well as the danger he poses to Harry.
The Time-Turner: Hermione uses a Time-Turner to go back in time and save Sirius Black and Buckbeak, a hippogriff wrongly sentenced to death. This leads to a series of events that change the course of the story.
Facing the Dementors: Harry, Ron, and Hermione confront a group of Dementors, using the Patronus Charm to defend themselves.
The Real Threat: It is revealed that Peter Pettigrew, who everyone thought was dead, is actually alive and has been living disguised as Ron's pet rat, Scabbers. Pettigrew is the one who betrayed Harry's parents, not Sirius Black.
The Finale: With the truth revealed and the real danger exposed, Harry, Ron, and Hermione confront Peter Pettigrew, saving Sirius Black in the process. The year ends with a sense of relief and newfound understanding.
Key Themes: The book explores themes of betrayal, friendship, justice, and confronting one's past. It also introduces new characters, magical creatures, and expands on the world of magic.