A young boy is out at sea near a beach. The beach is grounded, but no one sees that he's drowning. He goes under the water once more and starts to sink. Just as all seems lost he's saved by a mermaid.
After the opening theme, the setting has changed. The boy who was drowning is fine and riding on a train on the way to the Seto Inland Sea to visit with his grandmother. It's been so long since he has been here. He was so young at the time that he has no memories of it now.
Nagasumi arrives with his parents to his grandmother's home. She shows Nagasumi how the pray for his deceased grandfather. His mother cooks supper in the kitchen. They would of come earlier, but Nagasumi had supplementary lessons that finished only the other day.
We come back to the time where Nagasumi is drowning and is saved. He wakes up on a rock. It's sunset and his father is calling for him. Nagasumi swears he was drowning earlier. His father teases that he must of been saved by a mermaid. They head home and a young girl watches them in the distance.
Nights as fallen and the whole family is relaxed on the porch. Now Nagasumi's grandmother seem to be teasing him about being rescued by a mermaid. His grandmother does seem to believe it might be possible from the legends. She hints that there could be some trouble if he was saved by a mermaid.
As Nagasumi is describing the mermaid a young girl announces herself. She looks just like who Nagasumi is describing, but she has legs. Her name is San Seto. She suddenly asks to become Nagasumi's wife, and asks him to take responsibility for what happened that afternoon. This statement makes his parents think something seriously inappropriate happened and he's at fault.
The girl is now joined by thrill rather aggressive looking men. One has a rather cool appearance. A strong man type and wearing sunglasses at night. They are recognized as being Yakuza, but the Nagasumi's mother seems rather taken with the one in sunglasses. San's father is the boss of the Yazuka family and they have been requested to come back to the home of San. They are carried away. This seems to be the trouble Nagasumi's grandmother predicted.
The family is brought to a rocky cove. They are pushed off the cliff and into the water. Just as they think they have been executed, Sun is in the water. She now has her fish tail and is guiding Nagasumi to her home under the sea.
Nagasumi passes out. He hears a rough voice calling for his "son-in-law". He sees a rough looking man with a huge scar going across his face. Panicked he plays dead. Masa, the man with sunglasses, is ordered to perform CPR and locks lips with Nagasumi. Now he admits he's awake. He thinks his first kiss has been given to Masa. His mother is envious.
Now Nagasumi notices they are in a fancy Japanese style home that is set underwater. It's filled with Yakuza. San's father, Gouzaburou, introduces himself. Followed by her mother, Ren.
Gouzaburou explains one of the most important laws of the mermaid world. If a human were to discover the true identity of a mermaid. The punishment is execution. He's brought to tears, but Sun seems composed. He turns to angry why his daughter has to die for saving him. Gouzaburou tries to settle matters by killing Nagasumi and anyone else that knows their identity. Masa tries to hold him back.
San steps up and tells her father to stop. She saved Nagasumi knowing full well what could happen. She's prepared to die. This only seems to enrage her father more as he tries to get at Nagasumi and his family.
San's mother calmly approaches and tells Nagasumi of another option other then execution. The matter can also be resolved with they were to marry. It's not that he's against a marriage, but they don't even know each other yet. This hesitation enrages Gouzaburou. He wont allow his daughter to marry who he sees as so useless and he starts chasing him down.
Ren guards Nagasumi's parents. Gouzaburou orders Masa to kill Nagasumi, and a knife barely misses his head. He yanks open the door before the whole gang can attack him. The room is flooded with water. Everyone is now in a mermaid form. A huge shark nearly swallows him whole, but Nagasumi is saved by San. She takes his hand and they escape to the surface.
She tells him to run. She can't run since she has a fish tail. He lifts her out of the water and starts running. He refuses to let her die in his place. Eventually they make it out of the cave and back to the cove. He's exhausted.
Nagasumi finally asks San why she saved him knowing the rules it could mean her death. She says that if she were to abandon people in need she would disgrace the name of all the mermaids in Seto Inland. Another way of saying Chivalrous (ninkyou - 仁侠) is Mermaid (ningyo).
This moment is shattered by San's father. He's been discovered and the gang attacks. Before Gouzaurou can strike, Nagasumi asks to marry San.
He still refuses to allow the marriage and ties him to a boulder to trow in. His wife tells him to just let his daughter go.
Ending Theme: "Asu e no Hikari"(明日への光) by Asuka Hinoi; written, composed and arranged by Yugo Sasakura.