Xander understandably is not keen on being Spikes pet and mouths off and generally gives shit to Spike. Who is portrayed as a vampire very much in control, you like him but you don't and you certainly wouldn't trust him.
It all cluminates in Xander hitting Spike getting a beating then running away. At which point Spike looses the nice guy and Xander ends up hanging from his wrists, gagged for several days and then only allowed to crawl everywhere.
A dark little tale that always makes me cry but has a very nice ending (if you don't think too hard about Stockholm syndrome etc).
The relationship slowly improves after Xander gets sick and Spike reads a diary he had given to Xander to keep him occupied when things were better.
Buffy and Angel etc do find him and take him away but he chooses to return to his master.
Ideas on a postcard................ (joking! right here would be good)
EDIT: And a HUGE thanks to the speedy replys I got it is Dark Surprises by