Not plagiarism if credited.

I think there can be quite a bit of mileage in retelling old stories - heck, it's done all the time - and sometimes it can work with a specific set of characters like the Buffy ones. (I re-used - with accreditation - large parts of the plot and some of the actual text of a Burns/trad song for my Ballad of Jeannie the Red)

If it's just reproducing the entire text with different characters it's probably not very good (unless it's a really remarkable fit that no-one had noticed), but that's another matter.
Technically, if you are actually giving a direct quote rather than merely using an idea, you should use quote marks or otherwise make clear you're quoting. But that's academic style, and in prose you need things to flow differently.

Ideally I'd say that if you are reproducing a long passage of text you should include an A/N to the effect that you are doing this and identifying, as far as possible, where the passage is. But if someone's not done that, if they've merely said generally that it's based on XYZ or something, I still wouldn't call it plagiarism in a fandom context, I'd just suggest to the writer that they ought to include a more specific accreditation in future.

It's always going to be difficult when this sort of borderline case comes up. You have made the best judgement you can (and it seems reasonable to me) in a tricky case and have done your best to communicate this as reasonably as possible to all concerned. Maybe not everyone's happy with the outcome, but in no way does this make you the bad guy. It's what a good moderator should be doing.

*hugs*