A Truly Forgettable Narrative
No one remembers how this all started, and that's the problem.
The mind-altering virus known as "Eidetus" stripped the memories of all who were left on the surface while the elites escaped aboveground to their floating fortress. Thus, crazy powerful (but also crazy absent-minded) Psyphers roam the surface.
They can't retain memories, but they can whoop some butt. They have no idea where their keys are, but best believe they will blow you up with a fireball.
To stay alive and find the truth (but probably not those keys), you'll need to defeat the other Psyphers encroaching on your territory by channeling the memories you do have into psychic power.
You're on the hunt for your own memories, and no one better get in your way.