Harry is back with the Dursleys, where he sees on the Muggle news that a prisoner named Sirius Black has escaped. During Vernon’s sister Marge’s visit to Privet Drive, Harry accidentally inflates her after she makes rude comments about his parents. This forces him to pack his bags and leave the house. Just as he's realizing he has no way to connect with anyone in the wizarding world, he's picked up by the Knight Bus, an emergency travel service for stranded witches and wizards. He travels to Diagon Alley where he meets Cornelius Fudge, the Minister of Magic. Fudge tells Harry that he has to stay in Diagon Alley for the remaining three weeks before school starts again.
The night before the Hogwarts Express departs, Harry learns that Sirius Black is a convicted murderer, who is believed to want to murder Harry. The next day, a Dementor boards the Hogwarts Express, causing Harry to faint. Luckily, the new Defence Against the Dark Arts teacher Remus Lupin is there to fend it off. Harry, Ron, and Hermione learn that the Dementors will be patrolling around the school in order to protect Harry from Sirius.
Harry has several problems with the Dementors, including an event where during a quidditch match, he faints and falls off his broomstick 50 feet in the air. His broom is smashed by the Whomping Willow. Lupin, outside of class, teaches Harry to repel Dementors using the Patronus Charm.
Without a permission slip signed by a guardian, Harry is unable to participate in weekend excursions to Hogsmeade with the rest of the upperclassmen. George and Fred Weasley give Harry a Marauder’s Map, which details hidden passages in Hogwarts and live markers of the people who travel them. Harry uses the map to sneak to Hogsmeade where he overhears a conversation among faculty members about how Sirius Black was a family friend and godfather to Harry, who betrayed his parents and provided Voldemort with the location of their safehouse. Black was also convicted for killing thirteen Muggles and their friend Peter Pettigrew.
Ron and Hermione’s friendship suffers because Ron believes that Hermione’s cat, Crookshanks, ate his rat, Scabbers. At Christmas, Harry receives a state-of-the-art Firebolt broom as a gift, but there is no card or indication of who sent it. Fearing that it might be cursed, Hermione reports it to McGonagall.
Harry, Ron, and Hermione try to save Hagrid’s hippogriff Buckbeak from execution after it attacks Draco Malfoy. Their efforts are unsuccessful, but Scabbers reappears shortly after Buckbeak is executed.
Ron chases Scabbers, only to be attacked by a big black dog who drags him through a tunnel under the Whomping Willow into the Shrieking Shack. Harry and Hermione follow him to find Ron and Sirius, who is an Animagus and is able to transform into a dog at will. Lupin enters and explains that he is a werewolf, which led his friends James Potter, Peter Pettigrew, and Sirius Black to become Animagi. Lupin tells them that Scabbers is Pettigrew in his animal form, who is hiding from Sirius after he framed him for murdering Harry’s parents and thirteen Muggles.
Severus Snape arrives to apprehend Sirius, but Harry knocks him out. Lupin and Sirius transform Peter into his human form and prepare to kill him. Harry stops them, telling him that his father would have not wanted it.
At night, Lupin turns into a werewolf and becomes violent. Peter escapes again, but Sirius prevents Lupin from attacking the others. Dementors approach them and the three lose consciousness. When they wake up in the hospital, Harry, Ron, and Hermione learn that Sirius has been sentenced to receive the Dementor’s Kiss, which removes the soul of the recipient. Dumbledore advises Hermione to use her time-turner, to go back in time and save Buckbeak, who later carries Sirius away to safety.
