I know a lot of you know this story so I'll just give very basic info:
--It goes AU from season 4, I believe; Xander meets up with Spike in LA by random chance, and is promptly beaten up by thugs looking for Spike afterward
--Spike takes Xander back to his studio-ish apartment and has a woman that works for him take care of Xander
--Xander slowly heals while Spike does mysterious things (trying to get rid of the chip, I believe)
--A weird, pretty unhealthy relationship develops between the two of them
--It has two different endings
That should be enough for you, I'm sure :) And while I'm here I'll take the opportunity to thank all of you for your awesome writing, this wonderful pairing, and keeping BtVS alive forever :) Maybe one day I'll finish my first and only fic (which I'll post through this journal) and be able to give something back other than way-too-long and enthusiastic feedback =PP
ETA: Found! And duh: Modus Vivendi by wiseacress. Thanks!