Harry Potter and the Prisoner of Azkaban is the third book in the HP series. In it, Harry is getting ready to go to his third year at Hogwarts, when his evil aunt Marge arrives. Aunt Marge is Vernon's sister, so she's not related to Harry by blood, and she's really mean to Harry. One night, Harry gets really, really angry at her and loses control of his magic, making her expand bigger, and bigger. He's so angry in fact, that that night he runs away. He is picked up by the Knight Bus, where he finds out that Sirius Black, a dangerous murderer, has escaped from the wizard's prison Azkaban. He arrives at Diagon Alley, where the Minister for Magic, Cornelius Fudge, is waiting for him. It turns out they had found out Harry ran away, and had been searching for him. Conelius arranges for Harry to stay at The Leaky Cauldron, and a few days later Harry meets up with Ron and Hermione.

When they get on the school train, they sit in the same compartment as a new teacher at Hogwarts, Prof. Lupin, who is actually asleep when they get on the train. On the way to Hogwarts, the train stops, and a Dementor enters. A Dementor is a human-shaped thingy, that sucks the happiness out of a place, they are also the guards of Azkaban, and are searching for Sirius Black. When, it enters Harry kind of blacks out and hears screaming in his head. Prof. Lupin makes the Dementor go away and gives everyone some chocolate to make them feel better.

When the train arrives at school, they discover there are Dementors guarding it. They also learn that Prof. Lupin is their new DADA teacher, and that Hagrid is the new Care of Magical Creatures teacher. Sometime around Christmas, Harry discovers that Sirius Black is his godfather, and is the one who betrayed his parents so Voldemort could kill them.

One night, while Harry, Ron and Hermione are outside, Ron is kidnapped by Sirius Black. Harry and Hermione go in after him, and discover the truth about Sirius Black. If you want to find out what the truth is, as well as a few more details about the book, just READ IT;)