A small group of military personnel and survivalists dwell in an underground bunker as they seek to find a cure in a world overrun by zombies.

An eleven-person riot squad races to an insane asylum to quell a recent inmate outbreak and hostage situation. They arrive, only to be caught off guard by a sudden barrage of insane patients. When they've finally fought off a wave, two of their squad members are missing, their communications return only static, and the doors are locked. They're trapped. They break up into three teams to track down the lost members. Strange sounds and religious iconography suggest something beyond insanity has taken over the inmates. Sure enough, a simple rescue mission quickly morphs into relentless life or death battles with possessed. When McGahey, a trained, rational hostage negotiator, discovers the ringleader of this possessed insanity is none other than his brother, it falls on him to stop nothing less than the opening of a portal to Hell.