Four months ago, Alaric and Naia, two teenagers who'd never met, discovered they were living almost identical lives at alternate versions of Withern Rise, their family's riverside home. One day, they accidentally stranded themselves in the wrong realities. It's summer now, and heavy rains have caused the river to overflow. Withern Rise's grounds are under water. When Alaric and Naia find their separate ways into an earlier reality, they meet a boy called Aldous Underwood. Who is he? Could he be the old vagrant Naia has met in the present day? Or is he destined to die tragically young? And will their meeting change Underwood history?