Kelda has always protected her little sister Isla from danger on the rivers, and from the suspicions of their community. For Isla cannot breathe underwater - and so her very existence is forbidden by silvermens Lore. Now the rivers of England are growing sick: monstrous creatures are awakening and a fierce torrent of blame falls upon Keldas family. When betrayal comes, the sisters escape on a desperate journey upstream. Joining forces with a mysterious boy, Kelda discovers the darkest depths of her kinds secret history. But to save both her sister and the very life of Britains waterways, Kelda will have to make a sacrifice - one that will change her life forever.