There are three phases to a zombie apocalypse... run... hide...fight. Billy was there right at the beginning when the first victim coughed over their lunch. Amanda was working as a nurse at the E.R. and Samantha was about to meet the cult behind the whole outbreak. Professor Preston, head of the CDC, tried to rally the forces of science, but found himself awaiting his execution in a junkyard.
The list of characters grows as they run, hide, and learn to fight, not just the growlers, but the forces that unleashed them.
The government tried nuking the growlers, but that just spread the infection.
They tried sending in tanks, but the infection got inside.
They tried sending in the marines, now the growlers have body armor.