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
As the [Quidditch] match begins, [Harry] walks to his [detention] with [Snape], where he is forced to copy old records of misbehavior - specifically records of the [Marauders].
[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.
[Harry] and [Dumbledore] meet in the [entrance hall], and then head into [Hogsmeade] so they can safely [Apparate] to the [cave] where [Dumbledore] suspects the [Horcrux] is hidden.
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.