In The Vampire Slayer, Willow and Giles cast a spell in an attempt to heal Buffy from the trauma she is carrying around as the Slayer. But the spell ends up taking all of Buffy's Slayer powers and memories and transferring them to Willow! Now Willow is the Slayer, and Buffy is a normal person who remembers nothing of her past.
Willow, Giles, and Xander have been trying to come up with a fix, but nothing is working. So Willow has to go out and fight evil, even though she's had no training. All the while she's struggling with her magic, which seems to be getting more and more out of control. Faith arrives to help train Willow, and Xander ends up making friends with Spike in what started out as a way for Spike to learn about the Slayer, but what seems to be a genuine friendship by volume 2. But all of this is pushing Buffy away because she knows something is up, but no one is telling her what happened or what is going on (plus her friends are all busy with other people now).
Volume 2 ended with Buffy going missing because Hungrus the Slayer-Eater has captured her. Her friends didn't notice until 5 days later when they track her down at the park where she was captured. They're going to go after her and hopefully save her from Hungrus (who still went after her, even though she isn't the Slayer, so Giles took that as good news that the spell can be reversed).
I thought this was a super fun story, and I can't wait for more! Volume 3 is due out sometime next month and I can't wait!