The Golden Circlet

Lizzy Baxter's life is turned upside down when she moves to Cornwall and discovers she has a long-lost brother and that their mother is a mermaid. She soon learns that the tranquil underwater world is ruled by an evil mermaid queen, Taran, who will stop at nothing to complete her power by obtaining the black pearl, the key to ultimate power. Lizzy and her friends must defeat evil Queen Taran once and for all and free the rightful Queen from her prison. This is the fourth book in the series.