Thomasina and Anne are looking for adventure when their friend, Henry, disappears – snatched by a mysterious creature, and dragged into the depths of the River Thames. Determined to save him, the girls venture into the deep, and discover the enchanting Free Waters: a vast, glittering underwater city, home to mermaids and sirens. But the Free Waters are threatened by an evil Sea Queen. It is she who has Henry – and the girls must complete three treacherous and impossible magical tasks if they are to save his life...