Harry Potter and The Prisoner of Azkaban

At his third year in Hogwarts school, Harry Potter has to face the truth that a dangerous murderer, Sirius Black, escapes from Azkaban and about to look after him. 

Sirius even tries to enter the castle and cut off the Fat Lady picture. The surprising story appears, that Sirius Black is the one that caused Harry's Parents died.

On the other hand, a new teacher of Defense against the Dark Arts, Remus Lupin, gives Harry some important Spells to get away from the Dementors and other enemies. The good news is Harry can learn the spell well and fast.

Later, Harry accidentally sees Professor Trelawney, Divination teacher, gets into her trance. She predict that the Dark Lord's servant will return that night. In short, Harry, Ron and Hermione are visiting Hagrid when they see Ron's mouse, Scabbers, hiding there.

It bites Ron's hand and runs to the Whomping Willow where they see a grim. The Grim tries to attack and drag Ron to the root of Whomping Willow which lead them to the Shrieking Shack. But then it turns out that the Grim is Sirius Black, an animagus (a witch that can transform to be an animal). 

After a while, Lupin comes and hugs Sirius and they chase Scabbers which changes into Peter Pettigrew. It is clear that Peter is actually caused Harry's parents died, but he made it look like Sirius did. It ends with Sirius becomes Harry's godfather (although Sirius is still Azkaban's Most Wanted) and Peter Pettigrew escapes somewhere into the forbidden forest.