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 silvermen's Lore. Now the rivers of England are growing sick: monstrous creatures are awakening and a fierce torrent of blame falls upon Kelda's 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 kind's secret history. But to save both her sister and the very life of Britain's waterways, Kelda will have to make a sacrifice - one that will change her life forever.
