Along a lively river, in a village raised on stilts, lives a girl named Luna. All her life she has heard tales of the time before the dam appeared, when sprites danced in the currents and no one got the mysterious wasting illness from a mouthful of river water. These are just stories, though--no sensible person would believe in such things.

Beneath the waves is someone who might disagree. Perdita is a young water sprite, delighting in the wet splash and sparkle, and sad about the day her people will finally finish building their door to another world, in search of a place that humans have not yet discovered.

But when Luna's little sister falls ill with the river sickness, everyone knows she has only three weeks to live. Luna is determined to find a cure for her beloved sister, no matter what it takes. Even if that means believing in magic...