An outbreak of a pathogen nicknamed the Green Flu sweeps across Pennsylvania, causing extreme aggression, loss of higher brain function, and rapid mutation in its victims. Two weeks after the first infection, four immune survivors find themselves stranded together in the city of Fairfield: Bill, a Vietnam veteran; Zoey, a college student; Louis, a district account manager; and Francis, an outlaw biker. As the group fights through city streets, subways, and countryside in search of evacuation, they discover that the military's containment efforts are failing and that the infection is producing increasingly dangerous mutations in some of its hosts. Each rescue attempt leads to another setback, pushing the survivors further from safety and forcing them to rely entirely on each other to stay alive.