Harry left his negligent and abusive home to go to hogwarts.
Harry meets up with his friends and schoolmates Ron and Hermione and their pets scabbers the rat and crookshanks the cat. Harry gets attacked by dementors on the train over to the school but a teacher named mr lupian helped Harry and his friends
For months a prisoner from askaban has escaped named sirius black who is Harrys god father. Sirius black betrayed Harrys parents wall helping voldemort kill them. Mr Lupion talks to Harry about Sirius probably coming after him.
Wall Ron was trying to chase after his pet rat scabbers a black dog grabs Rons leg and drags him in a hole under the tree going after the rat. The hole was a passageway to a house
The black dog turns into sirius black and he and Lupion explains hes not here to kill Harry. Instead of Harry sirius is after Rons rat who is really Peter Pettigrew who was the one to betrayed Harrys parents.
When they leave the house Mr Lupion turns into a werewolf from the moon allowing Peter Pettigrew to get away. Sirius Harry Hermione and Ron escape and help Sirius get away from dementors on a Hippogriff.