The story takes place in season 6. Buffy and Spike are getting along, I can't remember if they're having sex or not. Giles and Willow (maybe Xander too) chase Spike out because they believe he isn't good for Buffy.
Buffy freaks out because she thinks he's dust. She has Tara do a ritual to find out who killed Spike and gets the memory of them chasing him out. She then kicks Willow and Giles out of her house I think.
This story wasn't very long when I read it but I really liked it. I can't remember what website it was on though.