Nerissa Marin hides among teens in her human form, waiting for the day she can claim her birthright—the undersea kingdom stolen from her the day her father was murdered. Blending in is her best weapon—until her father's betrayer confronts Nerissa and challenges her to a battle to the death on Nerissa's upcoming birthday—the day she comes of age.

Amid danger and the heartbreak of her missing mother, falling for a human boy is the last thing Nerissa should do. But Lo Seavon breaches her defenses and somehow becomes the only person she can count on to help her desperate search for her mother, a prisoner of Nerissa's mortal enemy. Is Lo the linchpin that might win Nerissa back her crown? Or will this mortal boy become the weakness that destroys her?