Sirius Black is portrayed as a murderous lunatic at the beginning of the third Harry Potter movie - The Prisoner of Azkaban. He's rattled the Wizarding World by giving the soul-sucking Dementors of Azkaban a slip and as the Knight-Bus conductor, Stan Shunpike puts it, is the only person to have done it. The most feared man in the third Harry Potter movie, Sirius is widely believed to have escaped from Azkaban to finish what his master, the Dark Lord couldn't thirteen years ago. While Harry is bewildered at the great hue and cry raised in the Wizarding World in the wake of the so-called felon's prison break, rest assured, Sirius deserves to be the poster boy of the good side in Harry Potter lore.
As Harry Potter and the Prisoner of Azkaban progresses, the Wizarding World's obsession with Sirius' great escape grows deeper. The Ministry of Magic staffer, Arthur Weasley thinks he's escaped because Harry still stands in You-Know-Who returning to power. "When you [Harry] stopped You-Know-Who, Black lost everything. But to this day, he still remains a faithful servant," he divulges to Harry. Amid the whole killer-on-the-loose narrative, Harry fails to understand why the Minister of Magic, Cornelius Fudge, and Weasley advise him to act responsibly. He fails to understand why Weasley warned him against seeking out the man who's escaped to kill him.
Sirius Black Exposes Peter Pettigrew's Identity
Still unbeknownst to Harry, Sirius Black was his late father's best friend, the best man at his parents' wedding, and his Godfather. In Harry Potter and the Prisoner of Azkaban, Harry gets to the bottom of how he figures into the whole Sirius' prison break situation. It turns out, when Lily and James went into hiding upon realizing they were marked for death, Sirius was among the few people who knew where they were. Branded as a traitor, the good side believed he sold James and Lily to Voldemort. Harry believed his godfather's betrayal led to his parents' death, and thus developed a deep-seated desire to avenge their fates throughout most of The Prisoner of Azkaban.
The truth eventually comes out in one of the gripping scenes of Harry Potter and the Prisoner of Azkaban - encounter at the Shrieking Shack. It is here the Trio learned that Sirius was framed by one of his best friends who was at school with him - Peter Pettigrew. Pettigrew was believed to be dead, and this lie lasted until Remus saw his name appear on the Marauder's Map. The duo exposed Pettigrew's identity to the Trio at the shack. Sirius was the person who spent twelve years in Azkaban without a trial and his is an extraordinary tale of justice denied and delayed. Pettigrew meanwhile disguised himself as a rat, and came into the possession of Weasleys.
In chapter 19 of Harry Potter and the Prisoner of Azkaban, "The Servant of Lord Voldemort," Sirius explains to the Trio why and how he broke out of Azkaban, and more importantly, how he was able to find out who and where Peter Pettigrew/ Wormtail (Scabbers to Weasleys) was. Black takes out a crumpled piece of paper from his robe, which turns out to be a photograph of the Weasley family that had appeared in the Daily Prophet the previous summer. He'd seen Scabbers on a boy's (Ron) shoulder and known at once. He'd put two and two together since the caption simply stated this boy (Ron) would be going back to Hogwarts where Harry Potter was. Additionally, Wormtail had a missing toe, which he cut off himself before he transformed in a ploy to fake his death. It's precisely why Cornelius Fudge said the only part left behind of Pettigrew was a finger.
Sirius Black's Prison Break
Chapter 19 of Harry Potter and the Prisoner of Azkaban delves into how Pettigrew faked his own death. Sirius Black cornered him on a busy street after learning he had betrayed James and Lily. The chapter reveals that sneaky Pettigrew blew apart the street with the wand from behind his back, killed people who were within twenty feet of him and went down the sewer. Hermione Granger's question, "If you don't mind me asking, how -- how did you get out of Azkaban if you didn't use Dark Magic?," silenced the commotion at the shack. Black answered her question by saying the only thing that kept his sanity, and powers intact was his innocence. When he was unable to cope with everything, he simply transformed in his cell, in that, he'd turn into his dog form. "Dementors can't see, you know..." he explained. Sirius informed the Trio that Dementors sense people by feeding off their emotions, and once he transformed into a dog, his feelings were less and less human.
While Sirius grew weak with time, and helpless without his wand, seeing Pettigrew in the picture with the Weasley family gave him new impetus. He was scared that Pettigrew was with Harry at Hogwarts and given that the Dark Side was beginning to reemerge, one night, he slipped past the Dementors when they opened his cell door to bring him food. "I was thin, very thin... thin enough to slip through the bars..." Sirius further stated that he swam as a dog to the mainland, traveled North, and entered Hogwarts grounds in his Animagus form. He'd been living in the forest, but he did come to watch his godson play Quidditch. At long last, Harry believed Sirius, and with that Pettigrew lost the game and fell to his knees. Sirius' escape bewildered the bigwigs in the third Harry Potter movie, and Chapter 19 of The Prisoner of Azkaban provides an in-depth insight into this great escape.