Start this by speaking with Cap 2099 in Nueva York. She's trapped in her mild-mannered cover identity, and doesn't seem to now what's up.
Follow her into town, and a Hydra ambush.
Smash that group of enemies, and then another before you run into Harry, who's able to re-activate her Captain America persona.
Smash through some more Hydra. You'll want to have someone that can deal with silver armor.
There will be one last big brawl at the Hydra lair, with more silver-armored goons, and Cap switching personalities back and forth. Still, these are minor complications- smash all foes to unlock Captain America 2099 for purchase.