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 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].
[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.
[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.