Underground, in the labyrinth of ancient tunnels and mine shafts, Lilly Caul and her motley crew of senior citizens, misfits, and children struggle to build a new life. But a secret ambition still burns inside her: she wants her beloved town of Woodbury back from the plague of walkers, and now the only thing that stands in her way currently roams the wasted backwaters of Georgia .

Way out in the hinterlands, amidst the rising tide of walkers pushing in from all directions, the psychotic Reverend Jeremiah Garlitz rebuilds his army of followers with a diabolical secret weapon.

Intending to destroy Lilly and her crew - the very people who vanquished his cultish church - he now has the means to bring a special brand of hell down upon the tunnel dwellers. The final confrontation between these two human factions unleashes an unthinkable weapon - forged from the monstrous hordes of undead, perfected by a madman, and soaked in the blood of innocents.