"...okay."

They make one last 270-degree left and Hermione knocks on an elegant wooden door. It's answered by an elderly, stern-faced woman in green robes, who says in a crisp Scottish accent, "Well, this is quite the group. What brings you all here?"

"The monster is a basilisk in the plumbing and also this clown memory-charmed Ron. Hi, I'm Cam."

"A basilisk? Are you certain? How did you come by this information?" She spares Lockhart a "what is wrong with you" glare, but he's clearly not the most pressing concern right now.

"...magic? It's a long story. I can't personally distinguish basilisks from any other sort of inappropriately large snake but it looks like this." He hands her a plastic basilisk. "To scale."

"Wandless conjuration? Without a complete visualization of the object? I find that rather hard to believe." She looks at the kids, then back at Cam. "Can you replicate the third piece of parchment from the top of the central stack on my desk?"

Cam sighs and hands her that too.

It's a subscription renewal form for a transfiguration journal. Her eyes go wide when she takes it. 

"Well. This is unprecedented. I have a lot of questions for you but first I must deal with the danger." Evacuating the school or sending everyone to their common rooms might tip off the Heir, and if they decided to go out in a blaze of glory the consequences could be disastrous, so, "Do you know who is controlling the monster?"

"No, I'm not confident it's a conjurable parameter but if you want a lot of little plastic basilisks we can pan through its history and see if it's been meeting anybody?"

"Unless any of you know where the Chamber of Secrets is, I don't immediately see a better idea. Come into my office." She steps aside to let them into a room balanced on the knife edge between "office of an extremely organized person" and "office of a person with too many jobs".

In they go. "Is he dangerous at all while I have his wand, I'm new to all this wand-related stuff," Cam asks, gesturing at Lockhart. "I can try for the Chamber but the plumbing doesn't obey conventional geometry so I might just wind up with a lot of self-interpolating Chamber parts."

"It's certainly worth the attempt. And if your magic works by proximity, you could try everyone who was near the basilisk at the time of the last attack." She gives a time and date.

"I did a model of Colin Creevy - I believe it's still where I originally made it, didn't want to carry it around - and there wasn't anyone else present, but I can pan back from that." Basilisk surroundings at that time and date.

He gets a number of unaccompanied plastic basilisks, which McGonagall vanishes as they accumulate, and eventually gets one being addressed by a redheaded girl with her hands positioned as though clutching something that didn't come through.

"Ginny?! No way she's the Heir! She's not!"

"I don't know what wizarding terrorism demographics look like but I wouldn't have guessed a little girl," acknowledges Cam.

"Yeah, I'm with Ron, his sister really didn't seem the type. And your whole family's in Gryffindor; shouldn't the Heir of Slytherin be a Slytherin?"

"Well, the legend is just that it's one of his descendants, but whoever it is has been attacking muggleborns, and I don't see why Ginny would do that."

"Nonetheless, mister Weasley, if she has been near the basilisk and survived she might know more than we do about the matter."

"If wizards do primogeniture it probably can't be a younger sister. Is she findable by conventional methods or do you need me for that?"

"She's probably in the Gryffindor common room at this hour. Miss Granger, will you go and fetch her while I sort out the matter of alleged memory charms?"

"Alright." Hermione sets off.

"Uh," says Cam, "basilisk, roaming the pipes? Is sending her off by herself wise?"

"It's no more dangerous than it was yesterday, but yes, perhaps we should stick together." She calls Hermione back in and examines the group. "Any further objections before we go find Miss Weasley?"

"I, ah, I must protest against these ridiculous accu--"