Eleven-year-old Lizzy has always known she was adopted,but knows nothing of her past. All she has is a locket witha curl of unusually pale blond hair inside-exactly the same as her own. One day on a boat trip the chain on her locket breaks and it falls into the sea-seemingly lost forever. The next day on the beach a strange boy hands it back to her, then disappears over the cliffs. When he turns up again, he entices her to follow him to his underwater world. Mesmerized by her own ability to breathe naturally underwater, Lizzy learns that the boy, Kes, is her twin brother and that she-like him-is half-human, half mermaid.