Vague storyline that bears little in common with the actual storyline... :-)
This is a very Buffy episode.
Buffy tries to fight a vampire, and nearly gets herself staked.
She's worried, Willow is hungry.
When learning about crystals, Buffy falls into a trance...
...so Giles slips her something.
Walking (what is Willow wearing?), Buffy goes to Cordelia's aid and gets pushed over.
Lots of tearfulness and stuff (we're cutting most of the episode here) as we discover it is a sacred slayer ritual. On her 18th birthday, a potion injected strips her of her powers, and she is to be locked up with a nasty vampire who she must defeat with skill and cunning.
So the plan goes to hell and back as the vamps get loose and kidnap Joyce. So, powerless, and helpless, Buffy goes to kill the vampire and get mom back. She knows what is happening, Giles broke and told her.
And, it goes without saying that she wins. Else it'd be the programme finalé!
Giles is fired from the watcher's council. He has become too close to Buffy and therefore cannot function properly. So says the stuffy Brit who needs to have his teeth knocked out. They reconcile, of a fashion.