Summary

Sapphire Trewhella's father suddenly goes missing one day during sailing off the coast of their home in Cornwall. After finding his boat, everyone proclaims him dead, but Sapphire and her older brother Conor are certain he's alive. A year later, Conor begins to disappear occasionally as well, and while searching for him one day along the beach, Sapphire meets a Mer boy named Faro, who draws her into the world of "Ingo", the alluring world under salty water. However, the Mer do not exactly hold humans in high regard, and Ingo turns dangerous when Sapphire's mother's new boyfriend, Roger, sets out to explore a place sacred to the Mer.