In [Harry]'s next [Occlumency] lesson, [Snape] sees Harry's memory of [Voldemort] and [Rookwood] - and gets angry with Harry for not closing his mind. As the lesson continues, Harry sees some of Snape's memories and then travels farther than ever before through the door in the [Department of Mysteries], but the lesson is interrupted by a scream and the two of them depart [Snape's office].
[Harry] and [Snape] rush to the [entrance hall], where they find an outraged [Professor Trelawney], having been sacked by [Umbridge]. Soon, though, [Dumbledore] arrives - ordering that Trelawney continue to live at [Hogwarts], and presenting Umbridge with a replacement [Divination] teacher - [Firenze].
Chapter 27 - "The Centaur and the Sneak" - In which Harry and his classmates have their first Divination lesson with Firenze, Marietta Edgecombe tells Umbridge of the D.A.'s meeting but suffers the effects of Hermione's jinx, Dobby warns Harry that the D.A. has been found out, and Harry denies the meetings to Fudge and Umbridge. Kingsley Shacklebolt modifies Marietta's memory, and Dumbledore takes the blame and leaves Hogwarts.
Divination with Firenze
At breakfast [Hermione] refuses to admit any desire to be back in [Divination], and [Lavender] and [Pavarti] talk about [Trelawney]'s misery. The group also wonders whether [Umbridge] will be spurred to be even more horrible after [Dumbledore]'s hiring of another part-human.
[Firenze]'s first [Divination] class is completely different from any of [Trelawney]'s - far less certain, but fascinating nevertheless.
As [Divination] class ends, [Firenze] asks [Harry] and [Ron] to pass a message on to [Hagrid], that his "attempt is not working," though he does not tell them what the attempt is.
After many attempts, [Harry] finally delivers [Firenze]'s message to [Hagrid], though Hagrid refuses to heed the warning.
As time passes, the fifth years get more and more nervous about their approaching [O.W.L.s], and the [D.A.] is the only thing that seems to keep [Harry] going.
The D.A. is Betrayed
A [D.A.] lesson on [Patronuses] is interrupted when [Dobby] arrives to warn them that [Umbridge] is coming, and the students scatter. [Harry], however, is caught by [Draco Malfoy], and Umbridge brings him up to [Dumbledore's office].
In [Dumbledore's office], [Cornelius Fudge] questions [Harry] and [Marietta] about the [D.A.], but the group is spared betrayal when [Kingsley Shacklebolt] casts a [memory charm] on Marietta, but Dumbledore takes the blame instead. When Fudge attempts to have him arrested, he instead knocks everyone out, warns Harry to continue working hard at [Occlumency], and escapes with [Fawkes] as [Phineas Nigellus] marvels at his style.