Harry starts off his third year at Hogwarts rather eventfully when he inadvertently blows up his Aunt Marge, goes on the run, and is then personally absolved by the Minister of Magic himself. He then learns that mass murderer, Sirius Black, is intent on killing him, and Mr. Weasley makes him promise a strange thing, that no matter what he hears he won’t go looking for Black. Confused, Harry agrees.
As he heads back to school armed with this knowledge, he learns that dementors, evil, soul-sucking creatures, have been stationed around Hogwarts to protect the students from Black. The dementors affect Harry much more than the other students because of his terrible past, and he quickly resolves to learn to defend himself when he begins collapsing every time they come near. He finds a willing teacher in Professor Lupin, the new Defense Against the Dark Arts professor, who begins teaching Harry the Patronus Charm, which is the only spell that will work against the foul creatures.
As the school year progresses, the third year students are buried under piles of homework. Hermione is already stressed out due to her enormous course load, and none of them can figure out how she’s getting to all her classes. It doesn’t help that Hermione’s cat, Crookshanks, repeatedly tries to eat Ron’s rat, Scabbers, causing the two friends to argue ceaselessly about the animals. They stop speaking entirely when it’s discovered that Crookshanks seems to have eaten Scabbers and Ron is devastated.
Harry has a few close calls with Black when he breaks into the castle, but Black is not apprehended. Things come to a climax as the school year ends when a number of events occur. First, Ron’s rat, Scabbers, turns out to still be alive and is really a wizard in disguise. This wizard, Peter Pettigrew, has chosen to hide himself in this way so that he can spy on Harry and his friends on behalf of Lord Voldemort. Harry learns that it was Peter, and not Sirius Black, that betrayed his parents to Voldemort. Just when Harry is sure Peter is finally going to be punished for his crimes, he escapes into the night.
Sirius, long thought to be a mass murderer and Voldemort supporter, turns out to be innocent. Harry also learns that Sirius is his godfather. He’s overjoyed when, for a moment, he thinks he might be finally free of the Dursleys and able to live with Sirius, but Sirius has to go on the run from the Ministry to avoid being apprehended.
Professor Lupin, Harry’s favorite teacher, ends up resigning his post when it’s discovered that he’s a werewolf. He narrowly avoids attacking the kids, and it’s Sirius that saves their lives when he turns into a huge shaggy dog to protect them.
It’s an action-packed year to be sure, but Harry is very happy that Sirius makes it out safely and is finally free from Azkaban. He’s comforted by the fact that he finally has a family-member, so to speak, out there watching over him.