Romi has been living happily ever since Heartsping became her best friend. But she and her mother, Belita, keep clashing over what Romi wants to do versus what she's told she must do... Then one day, an event that shakes the entire sea sweeps her mother away without a trace. Romi loses her mother without ever getting to say sorry — without one last hug. Standing before the sea for the very first time, she finds the courage to act, together with her best friend Heartsping, the sea boy Kite, and three Teeniepings... Together, they must face a dazzling underwater city and colossal creatures of the deep. And at last, Romi comes face to face with a secret her mother had never once told her.