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].
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.
Detention and an Overdue Kiss