Two By Two Hands of Blue by Thursday NextCrossover with Firefly. S7. After vanishing through a mysterious portal, Buffy, Spike and Willow find themselves aboard Serenity, where they and the crew find themselves forced into an uneasy alliance in order to retrieve a vital weapon in the fight against the First.
http://216.177.134.59/stories.php?go=chapters&no=4698 
