Lizzy Baxter's life is turned upside down when she moves to Cornwall and discovers that she has a long-lost brother and that their mother is a mermaid. Lizzy soon learns that the tranquil underwater world is ruled by an evil mermaid queen, Taran, who will stop at nothing to complete her power. After a terrifying battle, Taran is one step nearer to her goal. But then she disappears. Lizzy is suspicious. She knows Taran needs the black pearl - the key to ultimate power - but how is she planning to get it?