Day Four-As if a zombie apocalypse wasn't bad enough, the country is also in the throes of a second Civil War. Massachusetts has closed its borders to everyone, including the 101st Airborne Division and five million desperate refugees. It's an act of rebellion that the President won't stand for, and with nuclear missiles flying by the hundreds in China, he mulls the idea of using nukes on American soil for the first time.And his nuclear sites aren't just centered on Massachusetts, either. Anna Holloway and Lieutenant Eng are loose in Baltimore with vials of infected blood, the city of Philadelphia is crumbling under a terrific storm of undead, and half of Pennsylvania has fallen. Although the call to arms has been answered by thousands of men and women, it won't be enough. The fate of the country rests on a stout few: FBI Agent Katherine Pennock who has to hunt down Anna and Eng before they create more zombies, Sergeant Troy Ross of the 101st who finds himself the linchpin in an epic battle involving millions, and Doctor Thuy Lee who holds the key to a cure somewhere in her brilliant mind. Unfortunately she is still trapped in the Zone with legions of undead and if there is any hope of finding the cure, she first has to live through day four of the apocalypse.