Harry Potter and the Prisoner of Azkaban

It's Harry's third year at Hogwarts, but a shadow hangs over Hogwart's. A dangerous mass murderer, Sirius Black, has escaped the Azkaban Fortress - the Wizards' Prison. While learning to cope with the Dementors sent to protect Hogwart's, Harry learns the disturbing story of Sirius Black and yet more of his own history.