Moony, Wormtail, Padfoot, & Prongs
Shortly before Christmas, the Weasley twins give Harry their Marauder's Map, a magical document that shows every person's location within Hogwarts as well as secret passageways in and out of the castle. Harry uses a tunnel to sneak into Hogsmeade village where he overhears a disturbing conversation that Black was his parents' best friend and is his godfather and legal guardian. He was the Potters' Secret Keeper and he supposedly divulged the Potters' secret whereabouts to Lord Voldemort and murdered their friend Peter Pettigrew, as well as the twelve Muggle bystanders.
After Harry completes his Divination Exam, Professor Trelawney enters a trance and predicts that the Dark Lord's servant will return to him that night. Harry and Ron finally make peace with Hermione, but the trio soon learn that Buckbeak will be executed. When they visit Hagrid to console him, Scabbers appears, bites Ron, and Ron chases him to the Whomping Willow. A large dog attacks Ron and drags him and Scabbers into a hole at the tree's base. Harry and Hermione follow, finding a tunnel leading to the Shrieking Shack. Inside, Harry confronts Sirius Black, who, as an unregistered, and therefore illegal, Animagus, can transform into an animal at will. Lupin, who spotted the group on the Marauder's Map, suddenly bursts in and embraces his old friend Black. Confronted by Hermione, Lupin admits to being a werewolf and the Map's creator, along with Black, Pettigrew, and James Potter, the latter two also being illegal Animagi (a rat and a stag, respectively). Lupin and Black explain that Scabbers is actually Peter Pettigrew in his Animagus form. He is Voldemort's servant, and he betrayed the Potters, framing Black for the crimes. Harry is skeptical until Black and Lupin force Pettigrew back into his human form. Black explains he discovered that Pettigrew was still alive and escaped Azkaban to seek revenge. Harry stops Black and Lupin from murdering Pettigrew, believing that his father, James, would not have wanted his two best friends to become killers.
Lupin explains that Black was not the Secret Keeper. The Secret Keeper was actually Pettigrew who was able to convince people that Black was the traitor.