Book 3 starts out with the familar: Harry "causing" some disaster at the Dursleys. This time it happens to be accidentally blowing up the horrible Aunt Marge. Harry needs to get away very quickly, and is picked up by the Knight Bus, but not before he catches sight of a Grim (a giant scary dog) stalking him in the darkness. He also makes the unpleasant acquaintance of the Dementors, spooky creatures that are supposed to be on the lookout for the escapee from Azkaban, Sirius Black. The Dementors are horrible, and suck all joy out of their victims' bodies.
Safely at Hogwarts, the school year begins with a new class: Divination with Professor Trelauney, who promptly predicts Harry's iminent death! And there is again a new professor of the Dark Arts, Professor Lupin, who has some very strange habits.
The book is filled to the brim with magical drama: the Marauder's Map that helps Harry find his way to Hogsmeade Village for forbidden visits, the conflict between Crookshanks (Herminone's cat) and Scabbers (Ron's rat), the great Quidditch final, Herminone's big secret (which enables her to attend more classes than anyone else), the hippogriff (part horse, part bird) Buckbeak's death sentence, the appearance of Sirius Black who turns out to be Harry's godfather, and the discovery that Harry's father James was an Animagus and could turn into a stag. As always an exciting, whirlwind of a story!