Harry Potter spent the summer with the Dursleys before starting the third year at Hogwarts. Because he was too angry, he developed Aunt Marge and left the house, then bloated with the Minister of Magic. He is released away without any punishment, the main reason (which Harry can know later) is a serial killer, Sirius Black triumphs to escape from the magic prison, Azkaban to kill Harry. Black is a supporter of Voldemort and has been in prison for 12 years. This year, Azkaban's guards, known as Dementors, escorted Hogwarts. Harry and his comrades return to Hogwarts and each time they meet with Dementors, Harry is not happy because they force humans to recite their bitterest experiences and eliminate the happiness of their souls. The school received the new 'Self Defense Against the Dark Arts', Professor Remus Lupine, who was a great teacher.
Hagrid also began teaching subjects on 'Guarding of Magic Beings'. Their learning began as usual and Harry took several new subjects including 'Science of Knowledge', which throughout the year, Professor Trelawney often predicted Harry's death. But Hermione took more subjects than other students and sometimes it was as if she was taking 2 classes at the same time. Meanwhile, Harry could not go to Hogsmeade with his friends but he obtained the Marauder Map rather than Fred and George Weasley, which showed in detail Hogwarts and the surrounding environment. This helps Harry get to Hogsmeade through a secret pass.
Ron and Hermione always hold on to tendons, because they mainly involve their pets (Ron's rat, Scabbers and Hermione's cat, Crookshanks). Sirius Black broke into Hogwarts twice without successfully injuring Harry but anyone did not know how he did it. Harry can tell that Black is his father's best friend and he is also Harry's foster father. Sirius betrays Harry's father's mother by divulging the secret of their residence to Voldemort and causing them to be killed. Harry and his companions came to terms with Black in an amazing climax and got to know certain truths about the events that happened many years ago. They also get to know that Lupine is actually an invented wolf. Then, Harry and his companions fight with Dementors to save 2 innocent lives. In this third book, Rowling is better than the two books before this. He produced a dazzling story, combining magic, fantasy and betrayal, very extraordinary.