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.
With [Crabbe] dead but the [diadem] destroyed by the [Fiendfyre], [Harry], [Ron], and [Hermione] realize that [Nagini] is the only [Horcrux that remains.
Duellers come around the corner, interrupting the trio, and they soon discover it to be [Fred] and [Percy]. Even Percy is joking around while duelling, but a sudden explosion blasts them all backwards, and when they get up, the group discovers that Fred has been killed.