As the [Great Hall] empties out, Harry searches for Ron and Hermione, and [Kingsley] directs the school's battle plan. When McGonagall reminds Harry to search for the [diadem], though, he realizes his mistake and runs off to search.
As he runs from the [Great Hall], Harry thinks through what the final [Horcrux] could be, and where it could be hidden. With a sudden inspiration, he runs to find Nearly Headless Nick and ask who the [Ravenclaw] ghost is, with the intention of questioning her about the [diadem].
Harry runs down the [Grey Lady] and discovers that she is [Helena Ravenclaw], and that Voldemort managed, as a [Hogwarts] student, to find out from her the location of the lost [diadem]. Harry realizes that Voldemort must have hidden it in the school the night he asked [Dumbledore] for a job, and he runs off, thanking the Grey Lady and frantically trying to think where the [Horcrux] might be hidden.
Running through the corridors, Harry comes upon Hagrid and Grawp, who he learns have been hiding in the [cave] in [Hogsmeade]. He and Hagrid run off, but when Harry sees the demolished gargoyles outside the [staff room], he suddenly realizes where the [diadem] is hidden - in the [Room of Requirement].
The [Battle of Hogwarts] raging around him, Harry finally runs into Ron and Hermione, discovering that they've visited the [Chamber of Secrets] to retrieve the [basilisk] fangs and destroy [Hufflepuff's cup]. He tells them, then, that he knows where to find the [diadem], and they tear off for the [Room of Requirement] once more.
Back in the [Room of Requirement], Harry discovers Ginny, Tonks, and Augusta Longbottom, who all quickly depart. Ron stops for a moment, though, realizing the house-elves are still in harm's way - and for that he is rewarded with a heartfelt snog from Hermione. Harry does manage to pull them apart just long enough to realize that time is short, and they head back into the hallway to transform the room.
Back in the hallway, the fighting is continuing with the situation getting worse. Nevertheless, Harry lets himself, Ron, and Hermione back into the [Room of Requirement], this time as the room of hidden things.
Searching for the [diadem], Harry, Ron, and Hermione split up, but Harry soon instead runs into Draco Malfoy, Crabbe, and Goyle, intent on stopping him. A fight ensues, ending when a tower of flame from Crabbe's wand engulfs them all. Somehow, Harry manages to find [brooms], grab the [diadem], and get Ron, Hermione, Draco, and Goyle out, though Crabbe is left behind.