Following a car accident, Michelle (Mary Elizabeth Winstead) awakens in the basement fallout shelter of a conspiracy-crazed man (John Goodman) who claims to have saved her from the wreckage of the crash. He also informs her that a chemical attack has devastated the surrounding area, rendering the outside world uninhabitable. Emmett (John Gallagher Jr.), who barely made it into the shelter, corroborates this version of events, but Michelle remains suspicious. As the dynamics within the bunker continually shift, she begins to doubt the truth of both what's happening in the shelter and in the outside world.