Spike somehow ends up back in season 1 or 2, and he sets about changing things to make Buffy's life better. First being, preventing her from ever getting together with Angel. I don't know if he and Buffy are together yet, but somehow she finds out and is mad that he somehow stole her life and the PTB send her her memories of the original timeline or send Whistler with them.
Angel may or may not still lose his soul when Buffy tells him she loves him anyway and forgives him Angelus' misdeeds. (possibly this part is a different story) | 
