It's an AU. There are two dimensions, one is semi-cannon Sunnydale and the other is one in which, I believe, Wolfram and Hart took over. In any case, the latter is post apocalyptic. I don't remember how but some of the PA folks are brought to regular Sunnydale, friendships are made, there is mixing of dimensions and, as I recall, a semi-happy ending.
In the PA Sunnydale, Spike was a prisoner that Xander and co. help escape, Xander is a bad-ass, Cordelia is very strong AND helpful, Wesley has some type of mental handicap. In the end, Spike and Xander wind up together.
The only thing about regular Sunnydale that really stands out in my memory was Angel threatening to kill Giles if he ever gave chipped!Spike drugged blood again.
I've checked Spanderfiles and can't find it although I'm almost sure I read it there.
Any help is appreciated!