Somehow S.H.I.E.L.D. managed to figure out that Bucky was the Winter Soldier, then rescue and rehabilitate him. He's already a fully functional agent but he never fully regains his memories; he only knows what other people have said about his past. So when Steve wakes up from the ice, Bucky has to face the best friend he doesn't remember, but who loves him to death, and explain that he was once an ruthless assassin who killed a ton of people for Hydra, including their other friend, Howard. Angst from both sides.

Bonus: Steve and Bucky were once a couple, but kept it a secret from everyone. So future!Bucky has no idea that Steve is in love with him.