Title: Inverse Function
Author: ittykat
Rating: Teen.
Characters/Pairing: Gen, but Clint Barton/Natasha Romanov
Summary: AU: "I'm a soldier, not a spy." - Natasha once met a man who had a very specific skill set, who had earned a reputation. SHIELD sent her to kill him.

Written for my friend zenni, as it was inspired by her fantastic graphic

Found at AO3