Set in the UK in the immediate aftermath of a mysterious illness which swept the country and left millions dead, the series follows the trials facing a reluctant hero, Dan, and the group he forms around him. They must battle the elements, find sufficient supplies and equipment to survive, and protect themselves against the most destructive force on the planet: other people.

Steve, now recovered from his horrific injuries, forms a plan to restore the balance of power in their camp but his opposition is also playing a dangerous game. Meanwhile, Sanctuary reveals its secrets to Dan and the others, and they find safety and comfort behind the high walls. But safety is an illusion, and there are forces converging on them with a malevolence that they have not battled before.