Koshiro is recovering from breaking up with his girlfriend. On his way to work in the train, he first meets Nanoka while returning a name tag that she dropped as she was trying to exit the train. After returning to his house from work, he finds out from his father that his sister, whose face he can no longer remember, will be moving into their house for easy of communting to school. His girlfriend visits him during lunch break the next day to get back the key to her house. At the end of the work day, a customer gives Koshiro and his boss two tickets to the theme park next door. Upon leaving their work place, Koshiro meets Nanoka again and spends the rest of the day at theme park with her; they consol each other on their love lives. Their father appears at the end of the day to pick Nanoka up, revealing to both that they are siblings.

Koshiro is uncomfortable with Nanoka's doting presence as he struggles with his feelings for her. Nanoka takes over many of the households chores and even makes a bento for Koshiro. He reaches his limit when she arrives at his office to visit him, yelling at her, telling her to never visit him again and stop calling him Big Brother. To work off her anger Nanoka goes out with some friends to eat ice cream and karaoke until night. On her way back she calls once again, but is yelled at by Koshiro again. However she forgives him when he goes to pick her up on bicycle.

Unable to sleep, occupied with thoughts of Nanoka, Koshiro remembers how in his childhood what helped sleep on such sleepless nights. After he wakes up, he exchanges the bear he just brought with Nanoka's bear, both made by their mother. Her friend falls in love with a guy only to find that he is Tamaki's boyfriend. Upon returning home, Koshiro finds Nanoka taking pain killers, finding out later that she is having her period. On his day off the next day, it begans to rain, prompting him to wait for her with his umbrella.

While going to the movies with his sister, Koshiro is spotted by his Ex-girlfriend. Afraid that she got the wrong idea that he was trolling for high school girls, he meets her for dinner to explain.

Koshiro is shocked to find that Nanoka has emptied the trashcan in his room that contained the tissues of his guilt. He shouts at her for invading his privacy. That night he returns from drinking and drops and breaks her rice bowl when she surprises him contemplating it. Next day, when his father calls Koshiro at work and asks him to meet Nanoka at the station with an umbrella, he discovers that it is Nanoka's birthday.

Nanoka discovers that she has developed a reputation at school for having a brother complex. After working late on the upcoming school festival, she is walked home by a male schoolmate. Koshiro encounters them and shouts at her for being so incautious with boys. She protests her innocence and his jealousy shows when he blurts out that she has to be careful because she is cute. Next day she avoids him by leaving early and gets jostled and groped on the train. They have a talk that evening and make up. Next day, he makes a point of going with her and protects her on the train from the crowd.

Nanoka receives a love letter at school. She doesn't know what to do about it. She asks Koshiro what she should do about the love letter and he indifferently says she should make up her own mind and not consult him. She meets with her admirer and turns down his advances and then is disappointed when he doesn't seem to care. Koshiro regrets his prior indifference and tells Nanoka that if she wants advice about anything, she can ask him. She is impressed by his concern about her and says "I love you".

Koshiro contemplates moving out to resolve his situation while Nanoka plans to make him a sweater for Christmas.

Koshiro mentions to Kaname that he is thinking of moving out. She pushes him into doing it right away. Nanoka doesn't want to go home, so she arranges to stay overnight at her friend's house where she watches two normal sisters bicker and laugh. Koshiro and Nanoka make up on the phone. Koshiro announces to the family that he's moving out. Nanoka comes to his room and asks if they can sleep together. He agrees on the condition that they not meet again. She wakes up in the morning alone in an empty room and cries.

A month and a half after Koshiro has moved out, both the siblings are still thinking of each other, but keeping their promise not to see each other again. Nanoka sometimes walks by his new apartment or place of work, but avoids meeting him. Kaname has taken to sleeping over at Koshiro's place because "it's closer to work". One day when they are going to his place, they see Nanoka outside. He pushes Kaname away to hide from Nanoka, and Kaname guesses why he left home and is hiding from Nanoka. She confronts him, accuses him of perversion, and walks away. He stays home from work. When he opens the door to go to the store, Nanoka is standing there.

Nanoka breaks their promise and comes to Koshiro's apartment to see him. She offers to make him a meal, but his refrigerator is empty so he goes out to pick up some things. In the meantime, Kaname drops in and is shocked to find Nanoka there. She tries to make Nanoka go home by telling her that she is going out with Koshiro. But Nanoka refuses, saying that her feelings for her brother are greater and she insists that Kaname leave instead. Kaname encounters Koshiro on his way home and tries to talk sense into him, but he won't listen. Kaname says that she will not interfere any more. The siblings decide that they really do love each other and want to be together no matter what. They spend the night together as lovers.

Nanoka and Koshiro visit Mama together. After, sitting by the sea, Nanoka asks Koshiro if they should commit lover's suicide. He cries and says he doesn't know. She says that she is fine either way as long as she is with him. Koshiro quits his job. On his way out he meets Kaname, who wishes him well. Koshiro stands outside his father's house and says "Excuse me". The couple frolic in the playground to the disapproving stares of the local matrons. Then at night they sneak into the amusement park where they met. They sit on the ferris wheel and try to move it by prayer. It moves a bit. They carve their names on a tree and promise to go back there every year. When Nanoka has walked off to go home, Koshiro says "I love you".

Koshiro remembers the birth of his sister and their life together before their parent's divorce. Although they were too far apart in age to understand each other, they felt a vague sense of affection. In the present, Koshiro goes to see his mother for the first time in many years. She tells him how much Nanoka looked up to her big brother and tells him to be a good big brother to her. On the ride back, he remembers an incident when baby Nanoka gave him a glass of mud and he ended up keeping it because it made her so happy. He resolves to be a good big brother to her.