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. 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, the psychotic Reverend Jeremiah Garlitz rebuilds his army of followers with a diabolical secret weapon. He has designs on the destruction of Lilly and her crew and now, for the first time, he 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.