Pairing: Penny/Sheldon Cooper
Fic Title: Like the Rifle
Genre: Crossover, Action
Why This Must Be Read: I just can't say enough about how much I loved this fic. This story gives us a universe where Penny and Sheldon grew up on the road with Sheldon's militant father: hunting monsters, saving people, the family business (you know the drill), until Sheldon turned 18 and left for college. The fusion captures something really fundamentally powerful and aching about the characters as well as the the world building and mythos of Supernatural - Penny is her own brand of American cowboy, Sheldon is himself and so much more, and the rich background and case they investigate are just the icing on top.
This fic is locked on AO3 and can only be viewed by logged-in users with accounts, so if that doesn't work for you try here on DW instead. Also! Because this fic was written for a Big Bang, there's really fantastic art and fanmixes (more than one!) accompanying the story that you definitely need to check out as well.