When the whole world is drowning, who can you trust to pull you safely to shore? The coming years bring no salvation. In the UK, people flee the flooded coasts only to find drinking water rationed in the barren cities. Two girls in a Portsmouth care home find temporary solace in each other despite their differences – Mia is a withdrawn orphan and Lily a volatile tomboy rejected by her family. However, their friendship soon becomes too intense to last and they drift apart, finding their separate ways out of the sinking city. Years later, their paths collide in London at a meeting for Weather Underwater, a group of activists fighting the eco-fascist Ebbtide Party in power. The reunion forces both women to make terrible choices – with devastating consequences.