Harry is back at the Dursleys, where he sees on Muggle television that a prisoner named Sirius Black has escaped. Harry involuntarily inflates Aunt Marge when she comes to visit after she insults Harry and his parents. This leads to his running awayand being picked up by the Knight Bus. He travels to the Leaky Cauldron where he meets Cornelius Fudge, the Minister for Magic, who asks Harry to stay in Diagon Alley for the remaining three weeks before the start of the school year at Hogwarts.
The night before he is expected to leave for Hogwarts, Harry learns from Arthur Weasley that Sirius Black is a convicted murderer in the wizarding world, and he warns Harry that it is believed Black will attempt to murder Harry next. On the way to Hogwarts a Dementor boards the train, causing Harry to faint. Following the incident, Harry is helped by the new Defence Against the Dark Arts teacher Remus Lupin. Harry, Ron, and Hermione learn that the Dementors will be patrolling the school in an attempt to catch Black.
Later on, Lupin's Defence Against the Dark Arts sessions prove far better than those of Gilderoy Lockhart's (Harry's uselessly vain ex-teacher). They have a fun lesson on Boggarts (see Magical Creatures in Harry Potter) and then learn about more Dark Creatures. When Lupin supposedly falls ill, the much hated Potions Master Professor Snape temporarily takes over teaching Defence Against the Dark Arts and torments the class, much to their dismay.
At Hogwarts, Harry has several problems with the Dementors, including an episode at a Quidditch match during which he faints and falls off his broomstick from high in the air. His broom is blown away and smashed by the Whomping Willow. Working with Harry outside class, Lupin teaches him the Patronus Charm to repel Dementors.