This is a crossover with Doctor Who. Spike left at the end of the sixth season, however when he went to give Buffy what she deserves, it comes out a little more differently. Instead of getting his soul, someone from Spike's past comes forth.
There was many times that Spike had left Drusilla because he was off hunting Slayers. At the turn of the century (1899-1900), Spike ran into Rose Tyler from Doctor Who. Rose was hurt by the Doctor when he left her and Mickey to save Madame de Pompadour, when she encountered Spike. They had sex and he drank her blood and recongized her as a slayer.
In the present Rose gets pulled from the Alternate Reality. She encounters Spike, the Scoobies, the LA Gang. The Doctor with his latest companion-Donna. Martha and the Torchwood gang have tagged along to see what is going on in a small town called Sunnydale.
1. Flashes back to the times Rose and Spike interacted with each other. There must be at least three. Two of those have to be sexual in nature.
2. Buffy's been getting cryptic dreams about Spike's past.
3. Spike sleeping with Rose again in present time as a reaction to seeing Angel constantly around Buffy.
4. Buffy learning that Spike means a lot to her.
5. Rose uses the energy from the Heart of the Tardis to destroy the Hellmouth. (The energy never completely left her).
6. Rose calling Buffy out on her treatment of Spike in season 6 in front of everyone. Rose must reply to someone's question of how she knew that with "I see the whole of time and space. Everything that was, everything that is, and everything that could ever be."
7. SPUFFY ENDING!
3. Anya getting back together with Xander. (I'm sure she and Jack would get along just fine.)
4. Buffy getting it on with any other character other than Spike. She has to learn what it's like to be treated like she treated Spike.