Locklyn Adair thought that finding the Sea Enchantress would lift her family's curse. But the ramifications of her desire for a tail reach further than she could ever have dreamed. Now the entire Undersea Realm is in peril, as Circe, armed with the locket capable of controlling the Loch Ness monster, journeys to the North Sea in pursuit of revenge.

Darin Aalto's specialty is tracking down treasure. When it looks as though a magical locket has fallen into evil hands, he joins Locklyn and her crew on an expedition to recover it, hoping to find a key to his past identity along the way. Despite having no memory of the Mermaid leading the quest, he wonders if she was more than a friend all along.

As a vengeful enchantress, fiery monster, and impending war threaten the Undersea Realm, Locklyn and Darin must decide what they are willing to sacrifice in order to stop the locket's revenge.