Ron and Lavender finally break up, and so do Ginny and Dean.
Katie Bell returns to Hogwarts, completely mended, though she can't remember anything about who gave her the cursed opal necklace.
Everyone is gearing up for the final Quidditch match of the year.
Harry continues to check the Marauder's Map every chance he gets.
One day, he sees that Malfoy is up in Moaning Myrtle's bathroom.
Harry immediately goes to the bathroom in the hopes of overhearing Malfoy and Myrtle's conversation. Malfoy is terrified and sobbing.
When Malfoy discovers Harry, he attempts to hex Harry. Harry uses a spell he learned from the Half-Blood Prince (but which he had never used before). The spell proves to be incredibly dangerous and slashes Malfoy apart, causing blood to spill everywhere.
As Myrtle screams, Snape arrives on the scene and helps close Malfoy's wounds. Before he takes Malfoy to get medical care, he tells Harry to wait for him in the bathroom.
When Snape returns, we learn that Harry has used a bit of Dark Magic. Hoping to find out how Harry learned such a spell, Snape commands him to bring him every single one of his schoolbooks.
Scared to lose his Half-Blood Prince, Harry borrows Ron's Potions textbook instead. Snape doesn't seem to believe Harry's lie that Ron's book is really his, and sentences Harry to detention every Saturday for the rest of the school year.
This means that Harry won't be able to compete in the final Quidditch match of the year, taking place this Saturday.
As Harry is locked away in the dungeon for detention far from the Quidditch field on game day, he can't tell whether Gryffindor is winning or losing. When his detention ends, he returns to the Gryffindor common room to find the entire house celebrating: Gryffindor has won!
In a moment of elation, Harry kisses Ginny Weasley in front of the entire house, and so begins their romance.