When they find another person in cased in ice. They find out its actually (Y/n) (L/n) who went missing when Hydra kidnapped her because she was injected with the same super serum as Steve Rogers and then went missing after the plane crash with Steve. When she wakes up she remembers Steve and Bucky but not everything that happened to her.

You and your best friend Mari find yourselves stuck in what looks like an alternative universe, the mcu seems to be taking place around you, and while you know how things are going to happen...you cannot remember much about yourself. You have an identity to discover and a role to play in this universe, but will you find out something more complicated than being in a different universe? Or could it be that you belong there? *Based on Age of Ultron.*

It was only when he had almost lost her did he realize that it was her. It had always been her. She was the one that held his heart all along. (Steve's perspective will be in italics while the reader's are normal text. Song lyrics are in bold)

Steve Rogers was the supersoldier the whole world knew about, Y/N on the other hand, she was the supersoldier no one knew about. When she comes into the light she faces challenges hitherto undreamt of: the Battle of New York, Hydra's return, Ultron's rage, her team dividing, the infinity war, and the endgame. How will she survive it all? With the help of the person she loves most by her side through it all, after everything they've been through, their love will be timeless... I will follow the Marvel storyline, but with a few twists and turns of my own. Enjoy!