"Urgent appointment--couldn't be helped--"

"You're the Defense professor! You're supposed to be the best at fighting Dark creatures!"

"What about my sister! What about all that stuff you did in your books!"

"Well, you know, books can be misleading, simplifications for the reader and all that, had to make sure they'd sell decently, I may have fudged the truth in some places, or all the places, about who exactly did any given thing, and then done a few, or a lot, of Memory Charms so nobody could complicate the story . . ."

The things people will imperil their immortal souls over, if they haven't been raised with just laws and good examples to follow and the assurance that Heaven is achievable by ordinary people. It's tragic and a little embarrassing and not as immediately fixable as a monster terrorizing the populace.

"If he's actually a civilian we shouldn't drag him into combat. Let's go."

"Ah ah ah, not so fast, I can't have you three going around blabbing all my secrets either. Obli--"

"Expelliarmus!"

The wand Lockhart had been in the middle of drawing flies out of his hand. 

Ron grabs it and chucks it out the window."You're coming with us."

"What do you want me to do?" Lockhart asks, shaking. "I don't know where the Chamber is, I can't help you."

"We know where it is. Come on."

"Why do you want to bring him? We'd have to watch our backs against him as well as the basilisk. Let's tie him up and we can take him to the authorities when the immediate emergency is over." She starts pulling rope out of her bag of holding.

"Ah, yes, capital idea, that, very in favor of not coming along to fight a basilisk," says Lockhart, holding his hands out for more convenient tying-up.

This and the absence of his wand is probably sufficient to prevent him from sneaking up on them in ten minutes so sure, whatever.

And now it is time to go.

She also wants to gag Lockhart, because she understands how wizards work, but then she will follow the children and not stop to ponder the bizarre architecture. She doesn't have any hope that the boy's sister is alive, but catching the creature by surprise mid-meal would be nice and catching it before it before it leaves its lair again is important.

Here is a bathroom. It contains three toilets, three sinks, and a ghost, who explains when asked that she died upon seeing a pair of huge yellow eyes over that way.

Ghost: not immediately eating anyone's sister and therefore a problem for later.

This basilisk's gaze attack is a death effect: concerning! If the basilisks here are nastier than the ones at home this one might prove hard to solo. 

Its lair is in a secret passage inside the building: why is this building so large. Not important.

Harry finds the tap with the carving of a snake and hisses "open up" in a different language from the one he's been speaking so far. The secret passage opens up; it's a wide and very deep hole in the floor.

Samora peers into the hole. "That looks deeper than my remaining rope. We should try to lure the beast out instead of going down a hole we can't straightforwardly get back up."

"But Ginny could still be alive down there!"

"I'm sorry, mister--Weasley, right? Your sister has almost certainly already been eaten. The only thing we can do now is make sure the basilisk doesn't live long enough to kill anyone else."

"You're wrong. All the other victims were petrified and left where they were. If it took her into the Chamber there has to be a reason."

Hm, that's a good argument. Is it good enough that it's worth fighting on the monster's home ground with no line of retreat? Probably.

Typical adult behaviour.

Before Samora can finish her train of thought, Harry steps up to the edge of the hole. "I'm going in. You can come or you can stay here, I don't care."

The kid isn't bluffing. Samora jumps in the hole.