Gouzaburou is having a fantasy of better days with his daughter, Sun, when she was so small. That is until he remembers Nagasumi is now engaged to his daughter. He calls for a "liquidator" and someone unseen enters the room. An image of Nagasumi appears and it suddenly shot apart. This shadowy shooter swears to kill Nagasumi.
Later on a boat in the middle of the ocean, Nagasumi is traveling with Sun. He isn't ready to tell her, but she can sense something is wrong with him. Masa-san is driving the boat. Nagasumi has another of his fantasies. He's taking them to a deserted island for a date together. Sun is surprised that her father would arrange such a trip for them. Suddenly, a small voice announces that it must be because Gouzaburou finally accepts the relationship. A small girl wearing a conch shell is standing on Sun's shoulder. Introduced as Maki the Conch. She doesn't want to be a third wheel, but Nagasumi tells her it's fine.
Maki seems like the shy type. She becomes bashful as Nagasumi looks at her. As he's lost in fantasy, a rough and crude voice breaks his train of thought. This voice calls him a "sea louse". Though he can't find the source when he looks.
Masa-san leaves them and will return in the evening. he leaves with a warning to Nagasumi.
The beach doesn't have a single footprint on it. Nagasumi sees this has a chance to have a date without anyone trying to kill him.
He sees Maki struggling with the hot sand on her feet and lifts her to safety before the tide can strike her. As soon as Sun is gone. Maki's sweet personality changes and tells Nagasumi not to touch her. It was Maki that was the source of the voice earlier. Before Maki has a chance to strike, Sun appears and Maki's personality changes back.
Nagasumi isn't willing to give into Maki after surviving Sun's father.
Sun screams as a snake appears in front of her from a tree. Maki's conch shell on her back becomes a water powered machine gun. Several trees are destroyed. She looks to Sun and finds her clinging to Nagasumi. Angered, Maki screams about another snake. While Sun's eyes are closed in fear Maki starts shooting at Nagasumi. Eventually, Sun asks what Maki is firing at. Maki returns to her cute act.
Sun asks Nagasumi to put some sun block on her back for her. Maki objects and takes aim again. Before she can shoot, Nagasumi distracts Sun by saying there is a UFO. He grabs Maki and tosses her into the distance in the sea.
Nagasumi prepares to rub the oil on when Maki comes flying in on her shell like a jet. Striking him about the head.
Maki and Nagasumi are engaged in a battle of wills. Nagasumi relents and admits his intentions were not 100% pure about rubbing the sun block and lets Maki do it.
Maki is accidentally coated her whole body in sunblock and rubs her whole body over Sun's back. Nagasumi looks away and can only listen to the highly suggestive conversation that happens.
Later, Nagasumi realizes he needs to tell her about how they have to leave. Sun is in the water and has her fish tail. She tells him that she can't keep her human legs when wet....yet. She explains when a mermaid gets older. They can hold their human forms longer when wet.
Nagasumi mocks Maki's shell. Maki pulls the same trick he did earlier. She distracts Sun and strikes him. He's floating in the water and Maki is fake crying about how he made fun of her shell.
Maki suggests they dive to see the nearby coral reef. While Maki admires the coral. Sun is pulling him along with them. Nagasumi is bleeding from the ears from the depth of the pressure.
Back on the beach Sun apologizes for not realizing he can't handle the pressure as a mermaid can. Maki, in her false sweet tone, blames herself. Using this chance to point out how humans can't live underwater.
Maki volunteers to watch over him so Sun can go dry her legs. With Sun gone Maki is ready to finish him off with her shell gun. He declares his dedication to Sun's life if it costs him his own. Even Maki is impressed with the chivalry. Still, she takes aim and fires.
The shot was blocked by Sun's hand bag. She couldn't hear the whole conversation, but it was enough to understand. Maki cries as she apologizes to Sun, and sort of tosses an apology to Nagasumi as well.
The sun has set and they are traveling back on the boat. Masa-san is impressed to see him alive. Finally, Nagasumi tells Sun about how he has to return to Saitama tomorrow. Though he promises to return to her at the very next chance he can.
Sun points out that they will be living together. This is a shock to everyone present.
He tries to bring up the difficulties that they are still in junior high and changing schools is difficult.
Later in Gouzaburou's room, Maki asks him if it's true. He's so upset he's crying tears of blood.
At the station, Nagasumi's Grandmother and Sun's mother, Ren, are there to see them off.
Nagasumi notices how unsettled Sun looks. He thinks of what she is doing for him. Remembering what he's gone through so far. Thinking he's finally free of Sun's father. He looks out the train window to find Gozaburou clinging to the train. He casually closes the window cover and tells the curious Sun that it was nothing. Pounding on the window to knock him off the train. Nagasumi opens his bag to get some snacks for them. Inside is Maki. She fires a blow dart into his forehead. In her sweet tone she tells Sun she's coming to join them.
Ending Theme: "Asu e no Hikari"(明日への光) by Asuka Hinoi; written, composed, and arranged by Yugo Sasakura.