Twelve-year-old Isabella and her best friend, Griffin, have been keeping themselves and three younger children alive in Grimsdon since a massive wave flooded the city three years ago. The children eke out a living by searching abandoned homes and buildings for canned goods and useful items. Isabella, the leader, makes good use of her fighting skills (implausibly explained by school fencing lessons) to defend the group against predators. Junior genius Griffin raises their standard of living by engineering electricity from wave energy, enabling the children to power machines and grow vegetables on the roof of the abandoned mansion in which they live. But the children’s tenuous grip on survival is constantly threatened by sneaker waves, which swamp them without warning, by unstable buildings sinking into the floodwaters, and by the sinister Captain Sneddon, who runs a protection racket from a boat anchored in the swollen harbour.