Sometimes I Think I'm Not That Strong, But There's a Force That Carries Me On

After a botched assassination attempt on one of Natasha Romanoff's oldest friend, Yelena Belova, she immediately launches an investigation. Probing deeper and deeper into her own past, will she be able to face what she finds there? (set after I Would For You; interpretation of her Age of Ultron backstory)

Edit: now with angsty/cute BuckyNat epilogue!