A group of survivors, originally trapped together in a superstore by a series of escalating disasters, has split in two -- one group heading to Denver airport on a repaired school bus, the other remaining in the Sanctuary trying to rebuild the community they lost. But the world outside is dark and filled with dangerous chemicals that turn people into bloodthirsty monsters.

Trapped inside a chain superstore by an apocalyptic sequence of natural and human disasters, six high school kids from various popular and unpopular social groups struggle for survival while protecting a group of younger children.