After a very different outcome to WWII than the one history recorded, 1979 England is a country ruled by a government whose aims have sinister underpinnings and alliances. In the Hampshire countryside, 13-year-old triplets Vincent, Lawrence and William are the last remaining residents at the Captain Scott Home for Boys, where every day they must take medicine to protect themselves from a mysterious illness to which many of their friends have succumbed. The lucky ones who recover are allowed to move to Margate, a seaside resort of mythical proportions.

In nearby Exeter, 13-year-old Nancy lives a secluded life with her parents, who dote on her but never let her leave the house. As the triplets' lives begin to intersect with Nancy's, bringing to light a horrifying truth about their origins and their likely fate, the children must unite to escape - and survive.