The main character, Kuro, is a "bluefin tuna mermaid" that swims in the huge Fantra Aquarium.

He was surrounded by many friends and had a happy life watching his favorite keeper every day. But--

…… Suddenly, those happy days disappear as quickly as bubbles.

The world suddenly gets dark and Kuro loses his consciousness. At the same time, Kuro's friends began to suffer one after another and disappeared from the aquarium.

No one could stop the mysterious tragedy, and when Kuro woke up ... he was the only tuna left in the aquarium.

What happened to the huge aquarium?

Kuro begins to work hard to unravel the mystery with the help of his outsider friends.

[Translated from the official website]