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.

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 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".