Chapter 24 - "Sectumsempra" - In which Ron and Ginny are both single again, Katie Bell returns to Hogwarts but can't remember what happened to her, and Harry sees Malfoy with Moaning Myrtle on the Marauder's Map and runs to eavesdrop - but when Malfoy tries to hex Harry, Harry uses Sectumsempra on him without knowing what it does and slashes him apart. Snape heals Malfoy but gives Harry detention during the Quidditch match, which Gryffindor wins anyway. Then, when Harry arrives at the party, he finally kisses Ginny.
In [Charms] class, [Harry] tells [Ron] and [Hermione] about the [Horcruxes], [Ron] makes [Lavender] cry by brushing snow off of [Hermione]'s shoulders, and [Harry] learns that [Ginny] and [Dean] have had a row. [Professor Flitwick] then sets homework for [Ron] and [Harry], who have been so busy talking they haven't practiced the charm.
The Quidditch Final Approaches
[Harry], [Ron], and [Hermione] sit in the [common room], each thinking about recent developments in relationships. [Katie Bell] then returns, but can't remember the details of her [Imperius Curse], and the trio thinks very briefly of brewing more [Felix Felicis].
The [Gryffindor Quidditch Team] has a great practice thanks to [Katie]'s return and [Ginny]'s impersonations of [Ron].
As time passes, [Harry] continues thinking about [Ginny] and wondering whether [Ron] would be upset if they got together. Meanwhile, the whole school is focusing on the final [Quidditch] match of the year, which will determine the winner of the [Quidditch Cup], and [Harry] is still thinking about what [Malfoy] is up to and how to stop him.
Harry Versus Draco
[Harry] spots [Malfoy] in the bathroom with [Moaning Myrtle], and immediately [Draco] springs up to curse Harry; Harry then uses the [Sectumsempra] curse, springing gashes in Malfoy. [Myrtle] then freaks out and goes screaming to the whole school.
[Snape] rushes into the bathroom, quickly healing the gashes [Harry] has made in [Malfoy]. He sends [Harry] to get his textbooks, and when [Harry] returns, gives him [detention] every Saturday for the rest of the school year.
[Harry] runs and quickly borrows [Ron]'s [Advanced Potion-Making] and hides his own in the [Room of Requirement] before returning to show his books to [Snape].
Back in the [common room], [Hermione] refuses to lay off [Harry] as they talk about the [Half-Blood Prince], eventually prompting [Ginny] to yell at her as well.
Detention and an Overdue Kiss
With the [Quidditch] match approaching, [Gryffindors] are all quite frustrated with [Harry] for landing himself in [detention] with [Snape].