Buffy season 8, episodes #11-15, continues to tell the story of what happens in a world with 2000 full-strength Slayers. Here we have a single episode where the sinister secret organisation tries to dent Buffy's moral certitude. But the main story is a four-parter where a bunch of super-powerful Japanese vamps discover a way to wipe out all the Slayers. Buffy has to send Xander off to get some very powerful help indeed.
This does the usual mix of clever use of continuity, smart-mouthed one-liners, hilariously funny scenes, and heart-wrenching tragedy, in the way we have come to expect. And it moves the plot forward in significant directions.