Ohio high schooler Cassie Sullivan, armed with an M4 carbine, emerges from the woods to raid an abandoned gas station. Upon entering, she hears a voice calling for help. She finds a wounded man who points a gun at her, and then each of them asks the other to put their weapon down. When he pulls his other hand out from under his jacket, she mistakes the metallic glint of a Christian cross for that of a gun, so she kills him: the screen then cuts to black, and her backstory begins.

Earth is visited by an alien species that threatens humanity's existence. Events unfold in real time through the eyes of a handful of ordinary people across the globe as they struggle to make sense of the increasing chaos around them.