Chapter 33 - "Fight and Flight" - In which Hermione and Harry lead Umbridge into the Forbidden Forest, Umbridge insults the centaurs and they take her away, Grawp rescues Harry and Hermione from the centaurs, and Ron rejoins them with Neville, Ginny, and Luna, who insist on helping them in their fight. Luna suggests using the thestrals to get them to London.
The Aftermath of the Vision
Desperate to tell someone about his vision, Harry rushes to the [hospital wing] looking for Professor McGonagall, only to find she's at [St. Mungo's] instead. Harry, near panic because all members of the Order of the Phoenix are gone, rushes to find Ron and Hermione instead.
Harry finds Ron and Hermione, tells them of his vision of Sirius in the [Department of Mysteries], and grows impatient when they seem skeptical and unwilling to rush to [London] on the spur of the moment. Finally Hermione convinces him to check [number twelve, Grimmauld Place] for Sirius while she, Ron, Ginny, and Luna provide a distraction.
Harry runs to grab his [invisibility cloak] as the others plot to provide a distraction for [Umbridge], and when he returns they spring to action, clearing a corridor and running to head Umbridge off while Harry breaks into her office.
In Umbdridge's Office
[Umbridge] drags Harry back into her office, where he soon finds that his friends have been captured by the Inquisitorial Squad as well. She sends for Snape to ask for [Veritaserum], which he refuses to do, though Harry manages to cryptically deliver his message to him. Finally, with Umbridge ready to torture Harry, Hermione pretends to give in and promises to lead Umbridge to [Dumbledore]'s secret weapon.
Hermione leads a nervous Harry and an anxious [Umbridge] into the [Forbidden Forest], where they finally meet her target - the centaurs. Her plan doesn't work out quite as she hoped, though, as the centaurs try to charge her and Harry as well. They are spared, though, when Grawp arrives and drives the centaurs off with Umbridge in tow.
With [Umbridge] and Grawp gone, Harry and Hermione wonder what to do next - until Ron, Luna, Ginny, and Neville show up with their [wands]. Despite Harry's protests, once six [thestrals] arrive, attracted by Grawp's blood, Harry can no longer argue and all six take off on the horses for [London].
Chapter 34 - "The Department of Mysteries" - In which Harry, Ron, Hermione, Neville, Ginny, and Luna fly to London on thestrals, they are allowed access to the Ministry of Magic through the telephone booth, and they enter the Department of Mysteries - specifically the room containing small glass spheres, one of which is marked with Harry's name, and is later discovered to be a prophecy.
The group hops on the backs of the [thestrals] and rides to the [Ministry of Magic Headquarters] in search of Sirius. Harry leads his friends inside and to the [Department of Mysteries], through the phone box, the [atrium], the elevators, and the hallway past [Courtroom Ten], and inside.