... and though Joss Whedon didn't write it, it is steeped in the BuffyVerse. Every episode after this in season six seems stilted and awkward.
The episode really goes hand in hand with the previous episode, written by Joss Whedon, "Once More With Feeling". It was a musical episode in which everybody is compelled, by a magical spell, to spew out their feelings in song. Tara finds out Willow is manipulating her mind with magic spells, and considering that in Season five she lost her mind temporarily due to that season's villain, Glory, Tara is particularly hurt by this.
But this week, despite her promises, Willow is back at it and casts a spell to make Buffy forget her troubles and snap out of her depression. The explanation of this is too involved to get into here. The spell goes awry, though, and ends up wiping the memory of everybody at the Magic Box, which is the entire Scoobie gang and Spike. They awake not knowing who they are, and strangely enough almost figure it out, with a few humorous mistakes - Anya and Giles think they are engaged, Giles and Spike think that they are father and son, and Xander and Anya feel no connection to one another whatsoever, although they are the pair that is actually engaged.
It has lots of clever one liners and rewards for people who have followed this series throughout the years, but if you are just tuning in for the first time it is humorous enough to enjoy on its own. I don't think that Buffy ever recovered this level of natural flowing dialogue and humor for the rest of the series, although Buffy at its worst was always worth watching.