Chapter 25 - "The Seer Overheard" - In which Hermione finds out about Eileen Prince, Dumbledore asks Harry to come immediately, and on the way Harry runs into Trelawney, learning from her both that Malfoy is incredibly happy and that Snape was the one who overheard the prophecy. Dumbledore asks Harry to accompany him on a quest for the Horcrux, but first Harry confronts him again about Snape; finally, Harry runs off to fetch his invibility cloak, giving Ron and Hermione his map and Felix Felicis, and he and Dumbledore apparate to the seaside.
Detention and an Overdue Kiss
Harry returns to the Gryffindor common room to find a huge party celebrating the team's [Quidditch Cup] victory. Ginny runs toward him and they kiss, with Ron's implied approval, before heading out together to hang out by the lake.
Dumbledore's Summons and a Shocking Revelation
Sitting in the common room, Harry chats with Ginny, Ron, and Hermione about tattoos, Ron's approval of Harry's relationship with Ginny, and [Eileen Prince]. They are interrupted, though, when [Jimmy Peakes] shows up with a note asking Harry to go straight to Dumbledore's office.
On his way to Dumbledore's office, Harry runs into Professor Trelawney, who has just been kicked out of the Room of Requirement by an elated Draco Malfoy. In talking with her, Harry realizes that it was Snape who had overheard the [prophecy] - and sent Voldemort after the Potters.
Harry runs to Dumbledore's office and confronts him about Snape's role in the [prophecy]. [Dumbledore] explains his knowledge again, then tells Harry to fetch his Invisibility Cloak so they can go after one of Voldemort's Horcruxes.
Under the guise of fetching his Invisibility Cloak, Harry quickly tells Ron and Hermione where he's going and what he knows about Malfoy - and gives them the rest of his [Felix Felicis] for protection.
Chapter 26 - "The Cave" - In which Harry and Dumbledore enter the cave where Voldemort has hidden a Horcrux. After swimming to the entrance, Dumbledore spatters it with his blood to gain entry and they take a hidden boat across a lake. Protecting the Horcrux there is a potion that Dumbledore drinks, though Harry must force him to; after they obtain the locket Inferi erupt out of the water, and a weakened Dumbledore casts a fire spell to repel them as the pair flees the cave.
Harry and [Dumbledore] enter Voldemort's [cave] on the coast, breaking past several enchantments to reach the Horcrux. In the end, Harry has to force [Dumbledore] to swallow poisonous potion and they escape through an army of Inferi. When Harry realizes that [Dumbledore] is counting on him to get them back to Hogwarts, [Dumbledore] tells him that it is because he is with Harry that he feels safe.