Spike finds a card to a BDSM club of some sort at Xander's apartment and warns him off the place, telling him going there is a stupid and dangerous idea. Other stuff happens, they go to the club, and they end up in a relationship. They eventually do a scene where someone else cuts Xander and Spike drinks his blood. Spike can't, you see, because he's chipped.
Unfortunately, I can't remember anything else--not even where it was posted--which is endlessly frustrating because I've been looking for this fic off and on for over a year. Help?