A struggling older brother must escort his young sister across a desolate England to find safety with their father, after a mutating virus wipes out most of the population. Harry and Lily must survive against feral, infected humans, as well as the greater danger of other survivors. After Lily is injured in an animal trap, they seek refuge with a father and son pair, pushing Harry to put his trust in strangers. As Lily recovers, Harry struggles to decide between moving on to finding their father or staying in the seemingly safe home of Karl; the closer Lily gets to the pair, the more distrustful Harry becomes.