I've hunted and hunted through my bookmarks, but if I have this one tagged, the tags aren't ringing any bells. All I can remember is a couple of brief bits.
Spike is missing, and Xander asks Willow to perform a locator spell on him. She can't find him, so Xander thinks Spike dusted. For some reason, Angel takes control of Xan. (I don't know if Spike had claimed him, and Angel takes over the claim, or what.) Anyway, Spike comes back, and Xander's all confused about what to do - he wants to go with Spike, but either Angel won't let him, or Angel threatens Spike, or Xan thinks Spike doesn't want him. I remember Spike yelling at Angel that he was messing up Xander's head. And then (at some point) Spike ends up with Riley as a thrall (or something similar) and offers Angel a trade. Once he has Xander back, Spike makes Xander figure out what he did wrong by allowing Angel to take over. Something about Xan letting his demon rule him? Except I don't think Xander was a vamp, so -
Yeah, I could be confusing myself with more than one story. Sigh. Does this sound at all familiar to anyone?