In an AU season 6, Willow has accidentally sent Spike and Buffy off to another dimension where they are both ordinary humans and married. They are there for some time - long enough to develop a real relationship. A relationship that goes south for a while when they are back in Sunnydale, but that gets repaired when Buffy turns out to have brought a surprise back with her.
Buffy: The Vampire Slayer - Rated: M - English - Romance/Drama - Chapters: 1 - Words: 38,199 - Reviews: 22 - Favs: 148 - Follows: 24 - Published: 5/22/2013 - Buffy S., Spike - Complete