As soon as Alecto touches her Dark Mark, Harry can feel Voldemort approaching. Luna thinks fast and Stuns the Death Eater.
Harry and Luna hide under the cloak as puzzled Ravenclaws emerge from their rooms, confused by the spectacle before them.
Outside the common room, Harry can hear Amycus Carrow, Alecto's equally evil brother, demanding entrance. He's quarreling with Professor McGonagall, who calmly opens the door after giving him a hard time.
Amycus doesn't know what to do – Voldemort is coming, but there's no sign of Harry. He and his sister are going to be in deep trouble.
He decides to pass off the blame to the students, and say that they forced Alecto to summon Voldemort, then Stunned her.
Professor McGonagall won't stand for this. She and Amycus face off, and he spits at her. (Third grade much?)
This is too much for Harry, who rips off the Invisibility Cloak and inflicts the Cruciatus Curse upon Amycus – it's the first time he's successfully used it (or probably even thought to).
Professor McGonagall is shocked by the appearance of both Harry and Luna, and tells them to flee – Harry, however, wants only to know about the diadem. He knows it's too late to try and escape Voldemort, or to avoid him.
Professor McGonagall neatly ties up the Carrows, and confronts Harry about his plan. He tells her that he's acting on Dumbledore's orders, and she immediately springs into action; the Hogwarts teachers will join forces to defend the castle and put up protective spells, and hold Voldemort off as Harry searches for the diadem. The only problem is Snape…
As the three of them go off to discuss plans with the other Heads of House, Snape himself appears in the hallway. And he's ready for a fight.
Snape and McGonagall have a brief, cold confrontation, before breaking into duel mode; McGonagall is soon joined by her backup – Professors Flitwick, Sprout, and Slughorn. Overwhelmed by this onslaught, Snape leaps out the window and magically flies away.
Harry can see Voldemort at the Inferi-guarded island – in a moment he'll know that the locket Horcrux is gone, too. We can just see him puffing up now…
Professor McGonagall fills her colleagues in on the situation, and they make plans to evacuate their students and defend the castle.
As everyone hurries away to organize, Harry asks Professor Flitwick about the diadem, but he has no idea.
Professors McGonagall and Slughorn have a brief but significant confrontation, in which McGonagall coldly tells her uncertain colleague that Slytherin has to choose its loyalty – to Hogwarts, or to Voldemort.
The whole school is active as students move to the Great Hall, and Professor McGonagall rouses all of the castle's defenses, including some pretty awesome moving suits of armor.
Back in the Room of Requirement, all of Dumbledore's Army is mobilized – with some extra members, including Harry's old Quidditch team, and the Order of the Phoenix.
Harry tells them that the fight (or, should we say, Fight?) is on. There's a rush to get up to the Great Hall, where everyone is mobilizing. Only the Weasleys remain with Lupin in the Room of Requirement; Ginny wants to fight (and George and Fred are on her side), but everyone else thinks she's too young, including Harry. She resentfully agrees to stay.
The argument is interrupted by a late arrival: Percy Weasley. He and his estranged family face each other awkwardly (Fleur and Lupin attempt desperately to break the ice by talking about Lupin and Tonks's baby – always a good topic). Percy apologizes profusely for being pigheaded and stubborn; he's come to realize what was going on in the Ministry, but didn't know how to get out without being imprisoned. The Weasleys are all reunited.
Lupin suggests that Ginny should stay in the Room of Requirement so she can know what's going on, but not be endangered. Her parents agree to this plan.
Harry asks where Ron and Hermione are, but nobody knows for sure. Ginny says they mentioned something about a bathroom. Huh.
Harry's interrupted by a flash of pain from his scar – Voldemort is outside the castle, and he's ready to kill.