Takuro Mukojima is outside fishing when he reels up a girl who introduces herself as Muromi-san, a mermaid.

Muromi-san gives Takuro her cellphone number and is upset when he doesn't send any messages despite her sending him hundreds. Later, Takuro fishes up another mermaid who introduces herself as Levia-san (Leviathan), a giant sea monster from the Old Testament.

Levia-san, Hii-chan, and Sumida-san try to get Muromi-san to get over her hatred of marine mammals. She hates them because she had a boyfriend in the Mesozoic era who had developed lungs, but decided to stay in the ocean anyway. She thinks whales and dolphins are his descendants. Levia-san tells Muromi-san that that toothed whales returned to sea after having evolved into complete land mammals, which means her ex-boyfriend either evolved into something else or became extinct.

Takkun meets another one of Muromi's unique friends, later the trend continues much to the chagrin of Muromi.

Fuji use to admire Muromi and her gorgeous tail, but the current status of Muromi tail only brings anger.

After catching an unemployed fish, Muromi tells Takkun about the Ryuuguujou and the evil Otohime.

While river fishing, Taku can’t escape the salt water Muromi or her legendary friends.

Trapped by a gang of deadly sea animals, Muromi has only one chance of survival.

Youthful pride of Curiosity, how far will Taku go to learn more about the ancient civilizations that tried to defy the gods?

Ciel gets the chance to meet a unique cast of characters and later Yeti has to call on Takku for help.

Otohime has taken up multiple jobs like selling popsicles. She tells Tak-kun why she hates mermaids and her fall from the top. She tried to take what was left of her money and take a crack crack at the stock market, but that all fell through. It seems that her feud with Muromi is the result of a misunderstanding that is not easily cleared up. The two actually have nothing against one another. Takkun tries to help them out, but things don’t go too well on that front. A night of drinking together with Levia-san may have helped, but we may never know...

Fuji turns to the one person who might help her convince Muromi to regrow her tail fins.