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.
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 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.
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.
Detention in the Forbidden Forest