In the quiet Scottish seaside village of Dunlyre, Finn is enduring a winter holiday with his annoying new stepmother, wishing things could go back to how they were, while Sage is enjoying her new home, wishing things would stay as they are.
Finn has seen mysterious swimmers in the Firth late at night. Then, from the clifftop, first Sage, then Finn, hears an eerie song. Could the local legend of merfolk living amid the waves actually be true? When the new friends meet the magical sea-people, they are amazed and impressed, but the merfolk are hiding a secret. The two human children must put aside their own problems and help in the battle against the young merfolk's ancient underwater enemy before the last of their kind are lost forever.