Your sister with Natasha and Yelena and your also a widow you were a former red room assassin, but now your sister Natasha dies saving the world and you and Yelena fell apart you and Yelena grew apart with family and without you two knowing you both work for Valentina, so you become an assassin again. One day during work Valentina gave you mission to this vault to catch someone robbing her, so you go there and you get to the vault and you start fighting these random people(John Walker,Ava Starr, Yelena Belova, Bob) with the same mission as you (to kill each other) and Valentina locks the vault planning to burn you guys because you guys are “evidence” and you guys have no choice but to work together(over time you and Bucky have a slow burn)