Chapter 15 - "The Forbidden Forest" - In which Harry's popularity slips when his escapades lose his house 150 points, he overhears another conversation, and they serve detention with Hagrid who takes them to the Forbidden Forest in search of a wounded unicorn, which they find dead. Harry is rescued from Voldemort by a centaur.

Hagrid's Dragon
More time passes, and Norbert bites Ron on the hand. The trio hears from Charlie though, and Harry and Hermione plot to take Norbert to the Astronomy Tower at night to give him to Charlie's friends.
Ron's hand swells up, sending him to the hospital wing, where Malfoy visits to taunt him about the dragon. Then when Harry and Hermione visit, Ron realizes the note from Charlie - with details about how they were to get rid of Norbert - was in the book he gave Malfoy.
Harry and Hermione try to visit Hagrid, but as Norbert is getting violent, he can't let them into his [hut]. They do, however, tell him of the plan for Charlie's friends to pick the dragon up.
Harry and Hermione deliver Norbert to Charlie's friends at midnight, with the help of Harry's Invisibility Cloak. They overhear McGonagall catching Malfoy trying to turn them in, but their happiness is soon stifled when they forget the Invisibility Cloak themselves and are caught by Filch.
Hagrid's Dragon
An irate Professor McGonagall hands down huge punishments to Harry and Hermione, as well as to Neville, who had been trying to warn them.
Outcasts at Hogwarts
With 150 points gone from Gryffindor, Harry, and to a lesser extent Neville and Hermione, become the enemies of the school. Harry even offers to resign from the [Quidditch] team, as his teammates won't call him by name. The trio resigns to immersing themselves in studying for finals.
Harry overhears Quirrell talking with someone he thinks is Snape, and giving in. He rushes off to the library to alert Ron and Hermione.
Harry tells Ron and Hermione about overhearing Quirrell giving in, though as they're unwilling to get into more trouble, they resign themselves to being unable to do anything.
Detention in the Forbidden Forest
Harry, Neville, and Hermione get a note from McGonagall announcing their upcoming detentions.