As Harry, Ron, and Hermione walk sadly away from Hagrid's cabin, they have difficulties staying hidden under Harry's cloak because Scabbers will not stay still. Harry sees the reason for Scabbers' uneasiness once he spies Crookshanks slinking out of the Forbidden Forest, followed closely by the large black dog, who first tackles Harry and then Ron, ultimately dragging Ron away into the forest. Harry and Hermione dart after them and watch as the dog pulls Ron down inside the Whomping Willow, breaking his leg in the process. They follow him, but only after Crookshanks surprises them by pressing a hidden knot on the tree, causing it to quiet its branches. The path beneath the tree leads them into a boarded-up house that they instantly recognize as Hogsmeade's Shrieking Shack. In an upstairs room they find Ron, seated by Crookshanks, and guarded by Sirius Black, who the black dog has transformed into. Black disarms Harry and Hermione through the expelliarmus spell, and Harry, insane with fury at finally facing the man who caused his parents' death, leaps onto Black, reclaiming his wand with the help of Ron and Hermione, and ultimately forcing the unarmed Black into a corner, where he remains, at Harry's mercy. Crookshanks sits protectively on top of Black's chest.
Before Harry can gather his courage to kill Black, Professor Lupin bursts through the door, disarming Harry. Lupin quietly asks, "Where is he?" and Black points to Ron. The two men nod in silent agreement over something, and then they embrace. Hermione shrieks at Lupin that he is a traitor, and that she should have exposed him as a werewolf long ago. She says that she deduced his nature as a werewolf from seeing his Boggart turn into the moon, and from noting that he was out sick during each month's full moon. Lupin commends her cleverness, and begins to speak, only to be interrupted again and again by the accusations of Harry, Ron and Hermione. An exasperated Lupin gives each one of them back their wands, and explains that he was examining the Marauder's Map, watching Ron, Harry and Hermione exit Hagrid's cabin, when he saw the name Peter Pettigrew alongside Ron's. At that point, he realized that Peter must alive and disguised as Scabbers.
Ron is unwilling to believe that his faithful pet rat is really an animagus, a transformed version of a man named Peter Pettigrew. Black is furious and expresses his desire to kill Peter. Lupin stops him and demands that he explain everything first. Lupin begins saying that when he came to Hogwarts, a young werewolf, Dumbledore devised a way for him to leave Hogwarts secretly and safely through the Whomping Willow and remain, during the full moon, in the Shrieking Shack (thus explaining the name—the Shack was never haunted; just inhabited by a noisy werewolf). Lupin's good friends, James Potter, Sirius Black, and Peter Pettigrew, noticed his monthly absence and figured out his true identity. They then worked toward becoming animagi so they could assume animal form and keep Lupin company without placing themselves in danger. The four friends roamed everywhere and learned every part of Hogwarts and Hogsmeade, and they ultimately used this information to create the Marauder's Map.
Hermione points out that there have been only seven registered animagi over the century, and Lupin admits ruefully that they were unregistered; he then expresses his guilt for never having told Dumbledore that Black was an animagus and could have entered the castle in another form, but explained that he felt deeply grateful to Dumbledore for giving him an education and a job, that he didn't want to let him down. Lupin then explains that Snape, who now makes the revolutionary potion Wolfsbane that allows Lupin to keep his mind, thus remaining harmless during his transformation, as a student was always curious about the monthly disappearance. Once evening, Black played a prank by encouraging Snape to follow Lupin down the Whomping Willow, and James Potter ran after him and rescued him before he encountered the fully-grown werewolf at the other end of the tunnel. This trick is one of the reasons for Snape's grudge against Lupin, Black, James Potter and his son Harry. As Lupin finishes explaining this, Snape suddenly appears in the corner of the room, shedding Harry's invisibility cloak.
Harry is usually brave when facing enemies; he places his resisting hands on the face of Professor Quirrel in the first book, thus disintegrating his skin; he agrees to duel Tom Riddle, Voldemort's schoolboy shape, in the second; here, Harry attacks Black straight out, an unusual act of aggression for Harry. Perhaps he is emboldened by having his friends with him, or perhaps it is because he is still smoldering with anger over his parents' death at Black's hands. Whatever the reason, Harry pins Black to the ground quickly, preparing to kill him. This is unbelievable in principle, but within the context of events it makes a great deal of sense. Harry Potter grew up with a reputation of great competence against the Dark Arts, and instinctively when faced with them again, he knows what to do. Moments like this at the end of each story enable Harry to preserve this reputation.
During the dialogue with Lupin, many mysteries are explained, the most poignant of which involve Harry's father, James Potter, who was one of the cleverest students in the school, and a great friend to Black and Lupin. Hearing about James looking like Harry, playing Seeker for Gryffindor like Harry, or rescuing Snape from a werewolf just as Harry tried to rescue Ron from the black dog, brings a special meaning to Harry's reasons for being who he is. He is deeply proud of his parents, and Harry bears in mind who his parents were and what they would have wanted. This is the beginning of the point in the book where everything will be explained. Already, Harry's distrust of Black is countered by Lupin, whom Harry trusts, seeming to trust Black. Harry must trust his own good instincts in order decide for himself what to do and who to believe.