Concentrating hard on the idea that they needed somewhere comfortable to talk where they absolutely wouldn't be overheard, Hermione walked between the tapestry of the troll clubbing Barnaby the Barmy and the blank wall once, twice …

The third time, the door appeared. "Come on," she said, and pulled it open.

There was a corridor behind it, strung with floor-to-ceiling wind-chimes. They chimed sweetly but quite loudly as Hermione pushed her way through them.

"What are these for?" Ron asked, following her.

"Hermione told the Room she didn't want anyone eavesdropping on us," Luna said. "These must be in case anybody else in the school asks the room for somewhere to have a private conversation and gets in after us."

Ron got slightly tangled in a wind-chime. "Couldn't it just take them somewhere else?"

"I don't think that's how the Room works, Ron," Luna said.

"Remember, it took us to the same Room of Hidden Things as Draco," Hermione reminded him. She came to the end of the wind-chimes and stepped out into a small, cosy sitting room. There was a fireplace with a small fire, four comfortable armchairs, and a little table piled high with cakes of every possible description.

"Brilliant," Ron said, and helped himself.

Neville was the last to make his way through the chimes, and Hermione waited until everyone had taken a seat. "Right. Now. I think there's something going on."

"I think you're right," Luna said. "There's a new ghost, for one thing, and all the Nargles have fled the castle."

Hermione ignored the Nargles for the moment. "What ghost?"

"I don't know. No-one's seen it, even the other ghosts. But things have been moving around in the Potions classroom, at night."

"Well, that's just students, isn't it?" Ron said. He grinned at Hermione "Brewing Polyjuice potion, or something."

"In the summer?" Neville asked. "Is that why Professor Slughorn left? The ghost?"

"I don't think so," Luna said. "Hagrid said that he didn't really want to go, and that Professor McGonagall had called in a few favours to get him a teaching position at Durmstrang."

"She told me he was retiring," Hermione said.

Luna shook her head. "He's definitely not retiring. That's interesting, don't you think? That Minerva lied to you?"

"It is," Hermione said a little grimly. "When did all your job offers come?"

"Two weeks ago," Neville and Ron said in unison.

"Mine too."

"And me," Luna said.

"And it was two weeks ago that Ginny got an owl from Madam Hooch asking her to come and do some extra Quidditch coaching once term starts," Ron said.

"Do you think she wants us to deal with the ghost?" Neville asked.

Having a pretty good idea of who was really moving things around in the Potions classroom in the dead of night, Hermione shook her head. "No. I think … look, there's something I can't tell you, not yet. It's — it's not my right to. But there's definitely a reason Minerva has gone to a lot of trouble to get the band back together." At Ron's blank look. "It's a Muggle saying. I mean, it's Dumbledore's Army all over again. We were the ringleaders."

"And Harry. And Ginny," Ron said. "Who'll be here tomorrow. So if I guess what your secret is, will you tell me if I'm right?"

Hermione shook her head. "No, but if you find it out on your own you'll know."

"I'll Floo Harry tonight and tell him to make sure he packs the Marauder's Map," Ron said.

That'll do it, alright. Hermione could imagine Harry's face when he spotted Severus Snape on the map. I'd better warn him. For Harry's sake, if nothing else. "We all need to be prepared."

"I hope it isn't basilisks again," Neville said.

"I think if the school or the students were in real danger, Minerva would call in the Aurors or even the Department of Magical Law Enforcement," Hermione said.

"Dumbledore never did," Ron pointed out.

"Professor Dumbledore knew about the Rotfang Conspiracy," Luna said. "If he'd called in the Ministry, none of us would have got away with our teeth."

"Rotfang Conspiracy or not," Hermione said firmly, "the Ministry was definitely rotten in those days. It isn't now. Whatever Minerva wants us to do, I think we can rule out large-scale danger to the school."

"Why doesn't she just tell us?" Neville said.

"Maybe she will tomorrow, when Harry and Ginny get here," Ron said. "Maybe she's waiting to tell everybody together."

"Maybe," Hermione said. How close can I go without breaking my promise? "Maybe she can't tell us, not outright."

"There are curses that can do that," Ron said grimly. "Make it impossible for the victim to tell anyone what's happened to them." He stared at Hermione, eyes narrowed. "This thing you can't tell us …"

"It's not a curse, Ron," Hermione said quickly. "But I think it is to do with why Minerva wants us all here. I think it's why she didn't just ask us directly — I mean, she'd have to know that if she asked for our help we'd come."

"If it's not a curse —"

"Please don't ask me, Ron!" Hermione jumped to her feet and turned to give the fire and unnecessary poke, angry at herself for forgetting he was an experienced Auror now and trained to winkle secrets out of people. "I made someone a promise that I wouldn't tell anyone something. It's not something that hurts me, or anyone else, and I made it of my own free will, because the person I made it to had the right to want me to. I know you can probably trick me into telling you enough to work it out, and I don't want to break my word. Alright?"

There was a short silence. "Alright," Ron said. "But do you mind if I cast Finite Incantatem on you?"

Relieved, Hermione turned around. "No, of course not." She smiled at him. "Especially since you're going to do it anyway, Auror Weasley."

He returned the smile, but he still took out his wand. "Finite Incantatem! There. Still don't want to tell us?"

Hermione shook her head, and he shrugged. "Then it's your business. Alright. We need to wait for Harry and Ginny to start making a proper plan of attack, but here's what I think we should do tonight. Hermione, you head up to the —"

"Library," Hermione said, and everyone laughed. "Actually, I think Luna should come with me. There might be something useful in the back-issues of the papers, and she'd be better at looking for it than me."

Ron nodded. "Neville — you try and track down Nearly Headless Nick. He's always been the chattiest of the ghosts, and he'll remember you. Ask him for all the details he and anyone else incorporeal have on this new ghost in the dungeons."

Neville nodded. "And what are you going to do?"

Ron grinned. "I'm going to break into Professor McGonagall's office and have a chat to Albus Dumbledore."

Hermione looked at her watch. "You've got another two hours, then, before the password changes. Today's is 'gang aft agley'."

"Let's hope it's not an omen," Luna said softly.