Mukoujima Takurou is a lonely teenager who spends his time fishing at the pier and, to his incredible surprise, fishes up Muromi, a mermaid. Muromi first off doesn't realize she's a mermaid until she meets Takurou. Not only that, she is incredibly dense and crazy and has a drinking problem to top it off. Now every time Takurou goes fishing, Muromi appears and makes life interesting for him.
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.
Takku runs into Otohime selling popsicles, later he runs into Levia-san at a convenience store where an unexpected party breaks out.
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.
Fuji use to admire Muromi and her gorgeous tail, but the current status of Muromi tail only brings anger.
Takkun meets another one of Muromi's unique friends, later the trend continues much to the chagrin of Muromi.
After catching an unemployed fish, Muromi tells Takkun about the Ryuuguujou and the evil Otohime.
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.
Trapped by a gang of deadly sea animals, Muromi has only one chance of survival.
While river fishing, Taku can’t escape the salt water Muromi or her legendary friends.
Muromi is preparing a new years joke while Tak-kun is worried about his future. Muromi gives him some advice and in return, Tak-kun presents her with a present. Muromi then goes to enjoy the hot springs and watch the first sun rise of the new year with everyone.
Takuro Mukojima is outside fishing when he reels up a girl who introduces herself as Muromi-san, a mermaid.
Fuji turns to the one person who might help her convince Muromi to regrow her tail fins.