In which Harry Ron and Hermione start back to the castle, Crookshanks chases Scabbers, and a big black dog drags Ron into a tunnel under the Whomping Willow. Crookshanks stops the tree from attacking Harry and Hermione, and they follow the cat down the tunnel to emerge in the Shrieking Shack outside Hogsmeade.
They find Ron a captive of Black, who disarms them. Harry attacks Black, and with Ron and Hermione overpowers him, but Crookshanks stands between Harry and Black when Harry threatens to kill him. Lupin arrives and admits that he's Black's friend and a werewolf. It is revealed that Scabbers is an Animagus named Peter Pettigrew. Lupin tells the story of his becoming a werewolf and of his school days with James Potter, Sirius Black, and Severus Snape, the last of whom now appears.
Snape threatens Lupin and Black with the Dementors, Harry Ron and Hermione disarm Snape, Lupin and Black tell the truth about Harry's parents' deaths, Scabbers reverts back to Peter Pettigrew, the truth comes out about all the players in the drama, revealing that Pettigrew was responsible for the Potters' deaths, and Harry keeps Lupin and Black from killing Pettigrew.