I'm asking for someone else and didn't read that specific fic.
So, in S07 Buffy wants to go back in time to defeat the First. She has to take someone with her, Giles wants to go but she doesn't trust him anymore so she decides to go with Spike. Prior to the time-travel the team look into different dimensions/realities through some portals (a spell done by Willow) in order to find the best way to defeat the First.
Maybe a WIP, posted on Elysian Fields. The person who's looking for this fic doesn't remember in which season Spike and Buffy end up.
Thank you for your help!