Growing up with a single mother, Perce knows little about her father, who vanished when she was just a toddler. During her holidays, she and her cousin Peri spend their days exploring the woods behind her uncle's house and making dens in the trees. But when they encounter some mysterious visitors, they uncover secrets that their parents have been hiding from them both - secrets that reveal the truth about Perce's father and an incredible, otherworldly heritage.
As Perce struggles to come to terms with her ability to breathe underwater and her rightful place as the heir to an undersea throne, her aunt is kidnapped and she and Peri must race against time to rescue her. Along the way, they discover the true strength of their blended family and their ability to embrace both their land-dwelling and sea-faring cultures. Despite the traditions that seek to divide them, they fight to stay together and make peace with their past, bound by the unbreakable bond of love.