Professor McGonagall addresses the students who are gathered in the Great Hall, instructing them on how to evacuate. Students who are of age (seventeen) and want to fight are allowed to stay.
Her announcement is interrupted by an announcement of a different kind – Lord Voldemort's voice is amplified and rings throughout the castle.
He says that he doesn't want to fight them and lose valuable Wizarding blood. He'll leave them all unharmed if they turn over Harry.
Pansy Parkinson, an unsavory Slytherin in Harry's year, points out that the boy in question is right there – according to her, they should give him to Voldemort.
Professor McGonagall calmly and coldly tells Pansy to leave with the evacuating students. There's no way Harry's going to be handed over to Voldemort.
A bunch of older Ravenclaw and Hufflepuff students remain seated as the Hall is evacuated, as well as half of Gryffindor. Professor McGonagall shuffles the younger students out.
Kingsley Shacklebolt steps up to address everyone with the battle plan that the faculty and the Order have come up with.
Meanwhile, Professor McGonagall reminds Harry that he's there to look for something (he'd almost forgotten in the excitement) and he dashes off.
He can't figure out what to do for a moment – how could Voldemort have found the diadem? Professor Flitwick told him that it hadn't even been seen in living memory…
That phrase turns on a light bulb in Harry's mind, and he dashes off to find Nearly Headless Nick, the Gryffindor Tower ghost. Nick points him in the direction of the Grey Lady, the Ravenclaw Tower ghost. She sees Harry looking at her, and drifts away – way to be a pain, Grey Lady. Harry follows desperately.
Harry confronts the Lady, and asks her about the diadem. She says that she can't help him, but then agrees to tell him what she knows, in the spirit of defeating Voldemort.
The Grey Lady was apparently Rowena Ravenclaw's daughter, Helena; long, long ago, she stole her mother's famous wisdom-giving diadem in an attempt to become more clever than her famous mom. She then fled.
However, her mother became ill and longed to see her daughter one last time, so she sent a man who loved Helena to find her and bring her back to Hogwarts.
The man turned out to be another familiar ghost, the Bloody Baron. He found Helena, and when she refused to return, he killed her (what? We never do that to our crushes), then killed himself, full of remorse.
The diadem stayed where she'd hidden it, in a forest in Albania, for many centuries.
Harry isn't the only student she's told this story to – she also told it to young Tom Riddle, never guessing that he was so evil.
Harry quickly puts the pieces together. Voldemort got the story out of the Grey Lady, went to Albania, got the diadem, and then hid it somewhere in Hogwarts the night many years ago when he came back to ask Albus Dumbledore for a job.
As Harry runs off, wondering where the hiding place is, Hagrid comes flying through the window with his dog, Fang.
He'd been hiding in the hills, but heard Voldemort's announcement and returned to fight.
Harry and Hagrid both wonder where Ron and Hermione are, and rush off to figure it out.
They run past some shattered gargoyles, one of which mutters snidely at them.
Its ugly stone face reminds Harry of the bust of Ravenclaw he saw at Xenophilius Lovegood's house, then of the statue in the Ravenclaw common room – and then, of another statue from another time, a bust of an ugly old man, upon which Harry himself threw a wig and a tiara back in Book 6, Harry Potter and the Half-Blood Prince, in the Room of Hidden Things (the alter-ego of the Room of Requirement).
Wait a second! The tiara is Ravenclaw's diadem – and Harry runs off to find it.
Harry sprints by a group of students standing guard at a secret entrance, including Fred and Lee Jordan, then runs into Aberforth, who suggests that they should have kept some of the Slytherin students hostage – Harry tells him that Albus never would have done it.
They run off in opposite directions.
Finally, as Harry hurtles around a corner, he finds Ron and Hermione, who are laden down with basilisk fangs – they've been in the Chamber of Secrets all along, taking the fangs from the dead basilisk's skeleton to destroy the Horcruxes with.
It was Ron's idea; he even managed to speak enough Parseltongue to open the Chamber. Ron becomes more impressive by the minute.
Hermione destroyed Hufflepuff's cup with one of the fangs – one more Horcrux down.
Harry congratulates them, and tells them that he knows where to find the diadem too.
The three of them rush to the Room of Requirement, where they find Tonks, Ginny, and Neville's fierce grandmother.
They send all three of the women out (with strict instructions to Ginny to stay right outside and not join the fight).
Ron also has a last-minute revelation – they should warn the Hogwarts house-elves, so they don't all get slaughtered in the coming battle. This is too much for Hermione; overcome by emotion, she grabs Ron and kisses him (gosh, finally).
Harry awkwardly breaks up the emotional scene and reminds them that there's, um, still a Horcrux to find.
They step outside the Room to allow it to reform as the Room of Hidden Things. Out in the hallway, chaos has broken loose; Ginny and Tonks are shooting jinxes into the crowd, as Aberforth sprints by, leading a crowd of students into the fight. Tonks dashes off to find Lupin, who's dueling Dolohov, a dangerous Death Eater.
Harry thinks hard about the room he's looking for, and a door materializes in the hallway – they go in, and find themselves in the room of lost stuff. The three split up, looking for the bust of the old man with the wig and tiara.
The search is interrupted by some unwelcome guests just as Harry is reaching for the diadem – Malfoy, Crabbe, and Goyle. The terrible trio was waiting outside the Room of Requirement, and followed them inside.
Crabbe and Goyle, dumber than ever, oafishly taunt Harry.
Crabbe, finally ready for his spotlight, even turns against Malfoy, his old leader. Crabbe goes curse-happy, throwing jinxes everywhere, and tries to kill Hermione with the Killing Curse. Malfoy yells at Crabbe and Goyle not to – yeah, you heard right, he said not to – kill Harry, and Harry uses this distraction to Disarm Goyle.
Crabbe releases some kind of magical fire, which starts gobbling up the objects in the room.
It's clearly beyond Crabbe's control. The flames seem alive, and take on various monstrous shapes as they chase Malfoy, Crabbe, Goyle, Harry, Ron, and Hermione, who all sprint for the door.
Harry grabs a pair of broomsticks from a pile of junk and throws one to Ron; he swoops around, looking for Malfoy, Crabbe, and Goyle – he can't imagine dying in the fiery inferno.
He picks up Malfoy, and Ron and Hermione manage to rescue Goyle.
As they fly out, the fire tosses a few last charred objects into the air – including the diadem. Harry dives and catches it, then flies out the door.
In the hallway, the door to the Room disappears. The five of them sit on the floor recovering.
Crabbe perished inside the room, devoured by his own cursed flames.
Ginny, who was supposedly waiting outside, is nowhere to be seen.
Before they go looking for her, they peer at the diadem – it's bleeding a kind of tar, and suddenly, as Harry holds it, it screams and breaks apart.
According to Hermione, Crabbe's fire must have been Fiendfyre, a kind of cursed fire that's incredibly dangerous.
Hermione reminds them that now they just have to get Nagini the snake.
But she's interrupted by violent yells from the hallway. Death Eaters have gotten in, and are dueling with the castle defenders.
Fred and Percy are there, both dueling with masked, hooded Death Eaters.
Percy's opponent is revealed to be Pius Thicknesse, the Minister of Magic; Percy jokes that he's tendering his resignation. Fred's delighted that Percy's finally cracked a joke.
Fred is interrupted by a huge explosion – Harry flies through the air, and all is chaos.
When everything settles, Harry finds himself buried in wreckage, bleeding. Someone's blown a hole in the castle.
Harry hears a cry of utter agony, and it tears at his heart. He and Hermione pull themselves from the wreckage and see the three Weasleys on the ground. Percy and Ron are desperately clutching Fred's still body.