What I remember of the fic was that it was a Doctor Who crossover that took place in Season 4. Xander travels with the Doctor for a while before going back to his life in Sunnydale. I don't think we see what happens during the travels just that it happens. When Xander comes back he gets Spike dumped on him and he uses a screwdriver to set off Spike's chip.
That's really all I remember, I know it's not much. Any help is appreciated.