In the third book, called "Harry Potter and the Prisoner of Azkaban", Rowling uses time travel as the basis for the book. Harry learns that his parents were sold out to Lord Voldemort by their friend Peter Pettigrew, also accused of framing Harry's godfather Sirius Black for crimes he didn't make, locking him up in the wizarding prison, Azkaban. When Black escapes to find revenge, Harry and Hermione use a Time Turner to save him and a hippogriff named Buckbeak. Pettigrew, and the truth, escape from Sirius, causing him to be on the run from the authorities.