Chapter 21 - "The Unknowable Room" - In which Harry talks with Ron and Hermione about Apparition, Slughorn's memory, and Lavender, and Kreacher and Dobby show up to report on Malfoy, making Harry realize Draco's been using the Room of Requirement with Polyjuiced Crabbe and Goyle as lookouts. Harry spends some time trying to get into the Room of Requirement (unsuccessfully), Snape picks on him in class again, Moaning Myrtle reveals she's been talking with a boy, and Harry runs into a disheveled Tonks and wonders if she loved Sirius.
Harry's Fourth Lesson With Dumbledore
[Harry] arrives at [Dumbledore]'s office for his lesson and encounters an irate [Professor Trelawney].
[Harry] begins his fourth lesson with [Dumbledore], where [Dumbledore] soon expresses his disappointment with [Harry] for failing to complete his homework. [Dumbledore] then tells [Harry] about [Riddle]'s application to teach at [Hogwarts] and his taking of a job at [Borgin and Burkes].
In the [Pensieve], [Tom Riddle] visits [Hepzibah Smith] on business for [Borgin and Burkes], and she shows him [Slytherin's locket] and [Hufflepuff's cup] - two items that would very soon go missing.
[Dumbledore] tells [Harry] the rest of the story behind the [Hepziabah Smith] scene - including her death and [Riddle]'s framing of [Hokey]. He also points out its context in [Riddle]'s overall story, and then prepares to enter a second memory.
[Tom Riddle], now calling himself [Lord Voldemort], comes to [Hogwarts] to apply for the [Defence Against the Dark Arts] position, which [Dumbledore] refuses him.
[Dumbledore] briefly concludes his lesson with [Harry], mentioning that he knows which job [Voldemort] was after because the [Defence Against the Dark Arts] position has been cursed ever since [Voldemort]'s most recent application.
Sitting in the [common room], [Harry] tries to come up with a plan for getting the memory from [Slughorn], [Ron] frets about upcoming [Apparition] tests, and [Hermione] helps [Ron] fix his homework that went awry thanks to a faulty [Spell-Check Quill].
Trying to Catch Malfoy in the Room of Requirement
Then [Kreacher] and [Dobby] arrive unexpectedly in the [common room], telling [Harry] about [Malfoy]'s whereabouts and helping [Harry] realize that [Draco] has been using the [Room of Requirement] with [Polyjuiced] lookouts.
[Harry] goes to bed, wondering why [Malfoy] is using the [Room of Requirement] and planning ways to try to find out.