Ana (SARAH POLLEY) is a hospital worker whose life is shattered when zombies suddenly overrun her Wisconsin town. Having survived an attack that's left her husband as a zombie, she joins forces with policeman Kenneth (VING RHAMES), salesman Michael (JAKE WEBER), and Andre (MEKHI PHIFER) and Luda (INNA KOROBKINA) who are expecting their first child at any minute.
They seek refuge in an apparently abandoned shopping mall but then run into head security guard CJ (MICHAEL KELLY) and his minions Terry (KEVIN ZEGERS) and Bart (MICHAEL BARRY) who aren't pleased to have outsiders intruding upon their fortress.
With the zombies apparently unable to penetrate the mall's locked doors, the small group of survivors paints SOS symbols on the roof where they notice Andy (BRUCE BOHNE), a lone survivor on top of another building. When they realize that they're never going to receive any outside help, the group must decide what to do.
Other survivors then arrive at the mall, including the sarcastic Steve (TY BURRELL), dog lover Nicole (LINDY BOOTH), Tucker (BOYD BANKS), Frank (MATT FREWER) and others. When some of them turn out to be infected by zombie bites, the group realizes their security is jeopardized. From that point on and with the growing mass of zombies only intensifying outside the mall, the group must decide what to do next to survive.