Harry is back with the Dursleys, where he sees that a prisoner named Sirius Black has escaped. During Vernon’s sister, Marge’s visit to the house, Harry inflates her after saying rude comments about his parents. This leads to him running away and getting picked up by the Knight Bus. He travels to Diagon Alley, where he meets Cornelius Fudge, the Minister of Magic. He 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 that he wants to murder Harry. The next day, a Dementor boards the Hogwarts Express, causing Harry to faint, but the new Defence Against the Dark Arts teacher, Remus Lupin helps them. The trio 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.
Unable to visit Hogsmeade, George and Fred gives Harry a Marauder’s Map, which details hidden passages in Hogwarts and the person who is currently in them. Harry visits a village in Hogsmeade where he learns that Sirius Black was a family friend and godfather to Harry, who betrayed them and gave Voldemort access to their home. He is also convicted for killing thirteen Muggles in front of their friend Peter Pettigrew.
Ron and Hermione’s friendship suffers because Ron believed that Hermione’s cat, Crookshanks, ate his rat, Scabbers. At Christmas, Harry receives a late-model Firebolt broom, as a gift. Fearing that it might be cursed, Hermione reports it to McGonagall.
Harry, Ron, and Hermione tried to save Hagrid’s hippogriff, Buckbeak from execution, after attacking Draco Malfoy. Their efforts are unsuccessful, but Scabbers reappears shortly after hearing Buckbeak being executed.
Ron chases Scabbers, only to be attacked by a big black dog, who drags Ron through a tunnel under the Whomping Willow into the Shrieking Shack. Harry and Hermione follows him to find Ron and Sirius, who was transformed as a dog. Lupin enters and explains that he is a werewolf, which led his friends James Potter, Peter Pettigrew, and Sirius Black to becomes animagi. Lupin tells them that Scabbers is Pettigrew in his animal form, who was 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 prepared 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 advices Hermione to use her time-turner, to go back in time and save Buckbeak, who later carries Sirius back to safety.