Harry Potter and the Prisoner of Azkaban: Chapter Eighteen Summary & Analysis

After a moment of silence Ron says that Black and Lupin are crazy, and Harry points out that Black killed Peter Pettigrew twelve years ago. Black lunges for Scabbers, but Lupin holds him back and insists that Harry, Ron, and Hermione need to understand and have a right to know. Finally, Black stops struggling and agrees. Ron pulls himself up to leave but Lupin insists he stay and hold onto Peter. Black says that people didn't see what they thought they saw on the day that Peter "died."

In a trembling voice, Hermione says that Pettigrew can't be an Animagus because the Ministry keeps a register and would've known. Lupin laughs and tells Hermione that she's right, but there were once three unregistered Animagi at Hogwarts. He breaks off suddenly as the door opens. There's nobody there. Ron says that the shack is haunted, but Lupin says that the screams that used to come from it were his. He explains that he was bitten as a small boy and back then, the Wolfsbane Potion didn't exist. Dumbledore allowed Lupin to attend school and built the tunnel and the Shrieking Shack to keep people safe while he transformed. Dumbledore encouraged the rumors that the Shack was haunted.

Lupin says that his transformations were awful, but he had three friends—Sirius Black, Peter Pettigrew, and James Potter—all of whom eventually figured out where he went every month. Instead of deserting him, they spent three years figuring out how to become Animagi. They completed it in their fifth year and as animals, they could keep Lupin company since animals aren't in any danger around a werewolf. Because Sirius and James were such large animals, they could also keep Lupin in check and so they wandered the school grounds and around Hogsmeade. These explorations eventually culminated in the Marauder's Map.

Hermione interjects that what they did was extremely dangerous. Lupin heavily says she's right; there were several times when he nearly bit someone. He feels extremely guilty now, as he's been inadvertently helping Sirius all along. He says that he never told Dumbledore that Sirius was an Animagus, as that would mean admitting that he betrayed Dumbledore's trust as a boy. This is why Snape has been so nasty to him. Black is surprised to learn that Snape is a teacher, and Lupin explains that once, Sirius told Snape how to get into the Whomping Willow. James learned what happened and saved Snape, but Snape did see Lupin as a werewolf. Harry asks if Snape is upset now because he thought Lupin was in on the joke, and Snape himself emerges from under the Invisibility Cloak and affirms this.