When a violent storm leaves a mysterious Mer boy stranded in a Cornish rock pool, Morveren must navigate the dangerous intersection between the human world and the sea. As Morveren attempts to protect the stranger, she faces the growing distance between herself and her twin sister, Jenna, who remains skeptical of the truth. The narrative follows Morveren's internal and external struggle as she is drawn into the allure of the Mer world while tethered to her life on land. The story unfolds through a lyrical, atmospheric lens that emphasizes the physical sensation of the ocean and the emotional weight of shifting familial bonds.

This work is widely regarded for its evocative, lyrical prose that brings the underwater environment to life with sensory precision. Critics and readers alike note the author's ability to balance the grounded reality of Cornish life with the ethereal, dangerous allure of the Mer world. The narrative is frequently praised for its restraint and the psychological depth afforded to the protagonist's internal development. The book maintains a consistent reputation for being a sophisticated entry in the fantasy genre that avoids common tropes in favor of atmospheric storytelling.