The plot of the film, not the book : Harry's
Aunt Marge comes to stay and insults Harry's mother. He gets angry and she swells into a balloon and floats off. An angry Harry packs his case and leaves and sits on the pavement by a park. He see's a vicious black dog and falls back, the Knight Bus (a triple decker) picks him up and they ride crazily through London to the Leaky Cauldron. Harry speaks to the minister for magic who lets him off. Mr Weasley tells Harry a prisoner called Sirius Black is after him.
On the train to Hogwarts the trio enter a carriage where a strange cloaked man is sat. The train grinds to a halt and a dementor walks through the door, the last thing Harry sees is the man jump up ward off the dementor and he hears his mother screaming. Then he passes out.
He wakes up and they continue to Hogwarts. At Hogwarts the fat lady portrait is torn up by Sirius Black and everyone has to sleep in the Great Hall. Harry isn't allowed on school visits to Hogsmeade Village so he tries to sneak in his invisibility cloak. Fred and George stop him and give him the Marauders Map.
In Hogsmeade Harry overhears that Sirius betrayed his parents to Voldemort and killed a man called Peter Pettigrew.
In Hogwarts he looks at the Marauders Map and sees a shape called Peter Pettigrew walking around, he investigates but is caught by Snape. His new teacher comes to the rescue and covers for him.
Harry learns how to ward off the dementors with Professor Lupin.
Hagrid becomes a teacher and one of the beasts attacks Malfoy, who overreacts and gets the beast sentenced to death.
There new Divination teacher makes a freaky premonition about Voldemort.
The trio go to Hagrid's to comfort him and leave when a stone is thrown at them.
The beast is 'killed' and Hermione punches Malfoy.
Ron is dragged into the Whomping Willow by the vicious dog and Harry and Hermione follow. They meet Sirius who was the dog in Animagus form. Harry is about to kill him when Lupin walks in and knocks the wand out of his hand. They learn that Ron's pet rat is really Peter Pettigrew and he betrayed Harry's parents. Snape comes and Harry knocks him out with a spell.
They go outside and drag Pettigrew, the moon shines and they discover Lupin is a werewolf who attacks Sirius.
They escape and go to a lake where the dementors come. Harry and Sirius are close to death when a figure appears and wards them off.
Back in the hospital wing Harry discovers that Hermione has a necklace which can turn back time. They go back, save the hippogriff (Buckbeak, the beast which was going to be executed) and then Harry realises it was him who warded off the dementors and does it again. They then save Sirius from execution who flies away on Buckbeak.