Harry is in deep trouble. He has blown up his Aunt. He goes on the Knight Bus, a bus only for wizards. He thinks he is going to Azkaban, but Fudge says he won't be. Harry finds out why when he stays at the Weasleys. A mass murderer, Sirius Black, is after him! When he goes on the train to Hogwarts, he meets a dementor and a new professor. A dementor is a hideous creature that guards Azkaban. It is covered by black robes and has a completely black face.

At school, Hagrid is the new Care of Magical Creatures teacher. One of his lessons includes Hippogriffs. When Malfoy gets bitten by a hippogriff named Buckbeak, there is a trial. Harry tries to help him, but he gets into trouble on execution night. They are in Hagrid's house when they hear the thud of an ax and the wail of Hagrid. Hermione was shocked. "They did it, they really did it!" They see Hagrid crying, and they all go outside in the invisibility cloak.

Scabbers, Ron's rat, escapes so Ron goes after him. He heads for the tree, but a shaggy dog breaks his leg. In the tree, there is a trapdoor leading to the Shrieking Shack. Sirius Black is there. So is the new professor Lupin. Ron gives his rat to them. They cast a spell on Scabbers and the rat turns into a man! It is Peter Pettigrew, the real murderer who had framed Sirius. All of a sudden, Snape bursts in. All the wands at the same time do expelliarmus, and they stun him. Peter Pettigrew escapes. The dementors come and capture Sirius. Harry sees someone across the lake produce a perfect Patronus, a spell that wards off dementors. It looks like his dad. Then Harry blacked out.

Harry opened his eyes to find himself in the hospital wing. Hermione was awake, but Ron was still knocked out. Dumbledore came in and said to Hermione, "Three turns should do it. Be back by then. If you try, you might save two lives. I will lock the door." With that, Hermione took Harry downstairs. Harry was curious, but Hermione explained. She had gotten a time-turner from Professor McGonagall. It turns time around. Suddenly, she stopped dead in her tracks.

"Harry! We aren't only going to save Sirius. We're going to save Buckbeak." Hermione times it perfectly and unties the Hippogriff. Finally, Harry goes across the lake and waits for the thing that looks like his father to produce the perfect Patronus. But the person doesn't pop out of the bushes. Finally, it dawns on Harry. He saw himself!

"EXPECTO PATRONUM!" he yells, and all the dementors are chased away by a silvery stag that shot out of Harry's wand. Hermione, Harry and Buckbeak fly up to the tower where Sirius is being held and release him. They see him fly away. Then, they hurry back to the hospital wing and get in just before Dumbledore locks the door. All's well that ends well!