A mysterious necklace has been found in Liverpool by Ocean Jones, the owner of a local sightseeing company. She claims the necklace belongs to a real mer-person, promoting her boat tours with promise of a real sighting. The Secret Beast Club know that a mer-person's necklace holds powerful magic and wouldn't be carelessly lost. Feeling sure that Ocean isn't telling the whole truth, the Secret Beast Club vow to investigate. But this is their most dangerous mission yet - the sea is treacherous and mer-people have a natural mistrust of humans - will the Secret Beast Club save the day, and protect the mer-peoples' hidden home, before they too succumb to power of mer-magic?