“Merman’orandum” is the magical item, a symbol of the King of the Ocean. It is said that it records history since a thousand years ago. Also, it could enhance the physical ability of the owner and has the power to balance the aquatic ecosystem.

Now the Merman’s tribe is the King. They are elegant, powerful and possess Merman’orandum, while the octopus’ tribe, living at the Vicious Abyss. They are strong, arrogant, and striving to take revenge since it was the merman tribe that dethroned the Octopus tribe and exiled them to the Vicious Abyss. Keel, the next King of the ocean, joins the troop to investigate the Ring of Fire. Recently, he accidentally meets Dathan from the octopus tribe more often, which is unusual because there are no octopuses that dare to climb up from the abyss. They have been enemies since they were children and always fight against each other every time they meet.

Unfortunately, Keren, the current King, is ill due to a suspicious poison. So, he plans to give the throne to his son, Keel, on the next full moon day.

It is only Dathan and the octopus tribe who know the plan to raid the Underwater City.