So this film stars all the regulars from Harry Potter including Daniel Radcliffe, Rupert Grint, and Emma Watson. New faces to this installment were, as stated above, Gary Oldman, David Thewlis, and taking the place of Albus Dumbledore, Michael Gambon.
This sequel follows Harry as he discovers a man has broken out of the wizard prison, Azkaban and is believed to want to kill him. Harry discovers that the new Defence Against the Dark Arts teacher is a werewolf, along with the greatest teacher they'd ever had. Professor Lupin (Thewlis) teaches Harry how to create the Patronus charm after Harry realizes he faints anytime he sees a Dementor.
In the end, they find out that Ron's pet rat, Scabbers is really the one who is working for Lord Voldemort, rather than Sirius, the man who had escaped from Azkaban. Harry finds out that Sirius is his godfather and is welcome to stay with him anytime he wants. What Harry doesn't realize is that once Sirius is on his way, he'd be running away from the law for the rest of his life until his name is cleared.