Kasey Mizuhara is a sixteen-year-old STEM prodigy who moves through the world like a ghost, uninterested in people and devoted to cool, hard logic, except when it comes to her older sister Celia, the one person she loves without reservation. When Celia vanishes without explanation, Kasey refuses every official answer and sets out to find her using nothing but data and willpower.
Celia, meanwhile, wakes on a deserted island with no memory of how she got there, keeping company with a boy she names Hero, who remembers just as little about himself. As the world outside crumbles further under climate collapse, the sisters' parallel searches for each other force both of them to confront how much they actually know about who they are, and about each other.