Harry Potter is spending the summer with the Dursleys, before beginning his third year at Hogwarts. He blows up his aunt, Marge, in a fit of rage and leaves home, only to be met by the Minister of Magic himself. But he is let off without any punishment, the main reason (which Harry finds out later) being that a mass murderer, Sirius Black, has escaped from the wizard prison, Azkaban, to kill Harry, since Black was a supporter of Voldemort and has spent the last twelve years in prison. The Azkaban guards, known as the Dementors, are guarding Hogwarts this year. Harry and his friends return to Hogwarts and Harry has a horrible sensation whenever he faces the Dementors, since they force people to relive their worst memories and suck out all happiness. The school also has a new Defence Against the Dark Arts teacher, Professor Remus Lupin, who turns out to be a terrific teacher. Hagrid also starts teaching Care of Magical Creatures. Their lessons begin as usual and Harry is taking new subjects, including Divination, where Professor Trelawney keeps predicting Harry’s death all year. But Hermione is taking more subjects than the others, and appears to be taking two classes at once. Meanwhile, Harry can’t go into Hogsmeade with his friends, but he gets a map, the Marauder’s Map, from Fred and George Weasley, which shows every detail about Hogwarts and its grounds, helping Harry to reach Hogsmeade via a secret passage.
Ron and Hermione bicker the whole time, mainly due to their pets (Ron’s rat Scabbers and Hermione’s cat Crookshanks). Sirius Black breaks into Hogwarts twice, without managing to hurt Harry, but nobody has a clue how he does it. Harry finds out that Black was his father’s best friend and his own godfather, but he betrayed his parents’ whereabouts to Voldemort and was the reason that they got killed. Harry and his friends come face to face with Black in the enthralling climax and learn certain startling truths about the events, which took place so many years ago. They also learn that Lupin is in fact, a werewolf; Harry and his friends then fight the Dementors to save two innocent lives.
Rowling, in the third book of the series, surpasses her storytelling of the previous two books. She writes a totally gripping and enchanting tale of magic, fantasy and betrayal; truly phenomenal.