"It's in the Mirror of Atlantis," Harry said.
"We need Gillyweed," said Luna, "Lots of Gillyweed."
"The Mirror of Atlantis is located at the end of the third-floor corridor on the right-hand side," said Harry.
"What are you waiting for?" said Luna.
"You're not going to ask me how I know these things?" said Harry.
"Why would I do that?" said Luna.
"Because…because…" Harry searched for words to express the magnitude of the epistemic hole.
"You talk like a Muggle," said Luna, "You think like one too."
Harry puzzled over whether he had just been insulted. Luna skipped ahead to the third floor.
Harry and Luna flew a double-seated broomstick over the dieffenbachia and other unmaintained obstacles to the Mirror.
"You're a really good flier," said Luna, "I bet you're on the Quidditch team. What position do you play? No. Wait. Don't tell me. I bet you play Seeker."
"This Mirror lets you store an object until someone with the right intentions arrives," said Harry testily.
"I seek entry to the Ravenclaw Common Room," said Luna to the Mirror, "I want to sleep in a bed."
"In Gödel's name, what monstrously difficult riddle did the bronze eagle ask of you?" asked Harry.
"'Where is my diadem?'" said Luna.
"You're supposed to say 'lost' or 'hidden'," said Harry, "You're not expected to rediscover the Lost Diadem of Ravenclaw."
"Oh," said Luna.
"Since we're already here, let's give it a modicum of effort," Harry withdrew a mechanical stopwatch from his pouch and set it to five minutes.
"I want the diadem to save the world," Harry said.
"I aim to save the lives of all sentient beings," Harry said.
"I promise to bring you, Rowena Ravenclaw, back to life," Harry said.
"I seek to disassemble stars," Harry said.
Luna ignored him. Rowena Ravenclaw was not a jealous witch. If she concealed knowledge then it was not because the knowledge could be used for evil. It was because the knowledge itself was intrinsically dangerous. A cascade of logic began to self-assemble.
Rowena's basilisk attacks those who know it exists.
Luna forced her thoughts back into disarray.
"BRIIIIIING!" the mechanical alarm went off.
"I'm hunting nargles," Luna said.
The Luna in the Mirror held the Diadem of Ravenclaw in her left hand. The real Luna held it in her right.
"What is a nargle?" said Harry.
"You don't want to know," said Luna.
"What do you plan to do with this magical artifact of incredible power?" asked Harry.
"I'm going to get a comfy night's sleep in the Ravenclaw dormitory," said Luna.
"Can I, uh, try it on first?" Harry asked.
"Sure," said Luna. They were friends, after all.
Harry Potter thrust the diadem onto his head.
Then he had a seizure.
Harry's arms locked into place as if a powerful electric current flowed through them. Luna failed to pry the diadem loose. Luna dug through her satchel for the Comed-Tea can. She released Wanda.
"Help him," Luna said.
Harry's convulsions stopped. Luna wrestled the diadem off Harry's head and out of his hands.