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Harry Potter and the Prisoner of Azkaban
Sirius Black, a deranged prisoner and Harry's godfather, escapes Azkaban. He apparently wants to kill Harry, which brings the soul-sucking Dementors to Hogwarts to stand guard. In truth, Black, Professor Lupin, Peter Pettigrew and Harry's father were the best of friends in their school days, and it was Pettigrew who turned on the others to serve Voldemort. Black broke out of prison hoping to exact revenge on Pettigrew, who scrambles away from the group's clutches by morphing into a rat. Harry and Hermione help Black escape from the Dementors with the assistance of Dumbledore's Time-Turner.
THE SHORT VERSION: Harry meets his long lost godfather.