This is the third book in the Harry Potter Series. Sirius Black, a convicted murderer has escaped from the wizard prison, Azkaban and is coming after Harry. Sirius had been convicted of turning in Harry's parents to Voldemort and also for the murder of their friend Peter Pettigrew as well as 13 other muggles (non magical people). Harry discovers that Sirius Black, James Potter, Peter Pettigrew, and their current Defence against the Dark Arts teacher Remus Lupin used to be best friends. He learns that Sirius Black is innocent and that he is Harry's godfather. Harry and his friends set out to save Sirius from being recaptured and discover other truths along the way.
