When an army truck crashes outside a care home and infects the residents, four teenage friends – Charlie, Kelly, Finn and Steff – have to deal with the deadly consequences while also coping with their own family and relationship problems.

Charlie and Kelly deal with Kelly’s nan and Steff heads off on a dangerous quest with Finn. Kelly battles with her conflicting feelings about her nan. Billy is forced to go on the run. Michael welcomes a young boy, Kofi, to Dambury.

Kelly’s barely hanging on as her family falls apart. And Billy’s return ruffles feathers for hers. Frank and Cecily seek out others to recruit, and Cecily runs into old flame Morgan. A worried Finn, with Steff in tow, checks up on Morgan.

Finn is starting to feel the effects of the virus and the others take her to Morgan. He delays her growing symptoms and the friends are forced to sit their A Levels while dealing with betrayals that threaten to break them apart.

Having escaped the school carnage, an attack by the OAPs splits the gang in two. Charlie and Steff’s friendship is tested while Kelly and Finn help Morgan to create a vaccine. Billy takes control of the group who arrested her.

The gang race against the clock to save Dambury – but the army has other plans in mind. An infected Billy leads a revenge mission on the townspeople, exposing heartbreaking revelations that seek to destroy her family.