Chapter 6-24 Sectumsempra
- Harry tells Ron and Hermione about Slughorn and his memory. Ron split up with Lavender. Ginny and Dean too. Harry has conflicting feelings about liking Ginny but her being Ron's sister.
- Katie has returned but she tells Harry she doesn't know who gave her the necklace. Dean is off the Quidditch team now Katie is back.
- Quidditch match between Gryffindor and Ravenclaw is approaching.
- In the days before the match Harry keeps going to the Room of Requirement to find out what Draco is up to when he at some point finds him on the Marauder's Map in the boys' bathroom with Moaning Myrtle. He hurries over and listens at the door. Malfoy is crying and shaking, saying "..I can't do it, it won't work, and unless he does it soon, he'll kill me .." Next he spots Harry in a mirror. Malfoy draws his wand and a hex misses Harry by inches. Harry casts a ¤ Levicorpus but Draco blocks it. Another hex misses Harry, he tries a Leg-Locker Curse but misses. Halfway through a ¤ Crucio cast by Malfoy, Harry hits Malfoy with the ¤ Sectumsempra he read in the Prince's scribbles. As if slashed with an invisible sword, blood spurts out of Malfoy's face and chest and he collapses. At that moment Snape burst in the bathroom, goes over to Draco and with his wand starts working on the wounds.
- When the bleeding seems to have stopped Snape takes Malfoy to the hospital wing, and tells Harry to wait for him. When he returns he asks Harry where he got the spell. Harry lies it was in a Library book, but Snape does not believe him and with Legilimency he sees the copy of "Advanced Potion-Making". Harry is told to fetch his schoolbag and all his books.
- On impulse Harry asks Ron his copy of "Advanced Potion-Making" and goes to the Room of Requirement. Entering he is in a immense room with alleyways of objects hidden by past Hogwarts inhabitants. He hurries through the many alleyways, past the Vanishing Cabinet in which Montague got lost the previous year, and finally stopping at a large cupboard and hides his copy of the book in the cupboard.
- Then he goes back to Snape and hands over his books. Snape does not believe it is his copy and gives him detention for the rest of the term.
- Everybody seems to know what Harry has done. Harry also will be missing the Quidditch game due to detention. After all, Harry still defends the Prince, who didn't instruct him to use the spell but only made annotations for himself and might even have been used on him, and without whom Harry never would have won the Felix Felicis and saved Ron from poisoning.
- During detention Snape makes Harry sort old files for Filch of records of wrongdoers and punishments, and create copies if they are damaged or growing faint. He is to start with boxes one thousand and twelve to one thousand and fifty-six. These are full of cards with James Potter and Sirius Black, one card showing they got detention for hexing Bertram Aubrey.
- When he gets back to the common room, he finds out Gryffindor won the match. On impulse he kisses Ginny. Ron seems to not mind.