Over the last few months superhumans have been disappearing.  Anyone with superhuman powers or advanced technology is gone now.  The most notable people who are still around are Captain America, but without his shield, and King T'challa, who has secretly lost his Black Panther suit.  Even all of the refined Vibranium in Wakanda is gone.

There is a commotion outside, and when you go and look, you see a ship coming out of the sky.  With all of the Heroes, and even villains gone, there's no one left to fight the aliens.  It will be up to everyday people, including mutants and other empowered people, to stop them.