The story begins with the news that Sirius Black, a dangerous mass murderer and loyal follower of Lord Voldemort, has escaped from the wizarding prison of Azkaban. He is rumored to be hunting Harry, so the school is surrounded by Dementors—nightmarish guards that feed on human happiness and force people to relive their worst memories.
Harry Potter is an orphan living a miserable life with his cruel aunt, uncle, and cousin, the Dursleys. He has no idea about his true heritage until, on his eleventh birthday, a half-giant named Rubeus Hagrid bursts into his life with life-changing news: Harry is a wizard, famous in the magical world for surviving an attack by the dark wizard Lord Voldemort when he was a baby.
Harry’s second year at Hogwarts begins with a strange warning from a house-elf named Dobby, who tries to prevent Harry from returning to school, claiming that a terrible plot is underway. Despite Dobby's interference, Harry returns to Hogwarts, where he begins to hear a mysterious, cold voice whispering through the walls that only he can hear.
After the traumatic events of the previous year (the return of Voldemort), Harry is treated as a pariah. The Ministry of Magic, led by Cornelius Fudge, launches a smear campaign, labeling Harry a liar and Dumbledore a troublemaker. The school is infiltrated by Dolores Umbridge, a Ministry official who bans practical defense training and imposes a draconian regime on Hogwarts.