After the sudden death of their parents, Haruka and his twin sister Sora move to their grandparents' house in a small village they often visited as children. There, Haruka encounters several of the residents, some of whom are old friends, but Sora stays at home, becoming more and more distant, refusing to go to school, even as Haruka starts attending classes.

Sora finally starts attending school with Haruka, though she is disappointed to find that she and Haruka are in different classes. While cleaning the swimming pool as part of a mixed gym, Akira accidentally soaks Kazuha and takes her to dry off. Haruka finds Kazuha's phone and goes to return it, only to overhear a conversation with the hint of a secret. Does Haruka misinterpret the relationship?

When Kazuha realizes that Haruka has overheard her conversatoin with Akira, she attempts to clear up any misunderstanding. She reveals that Akira is, in fact, her illegitimate older sister, born of a different mother. Due to her family's societal standing, Kazuha's father is not able to acknowledge Akira as his daughter, causing Kazuha to be resentful of her father and to feel a burden of guilt and responsibility towards the always-hard-working Akira.

After her collapse from exhaustion, Akira confronts Kazuha about her inappropriate sense of guilt she feels regarding their father's actions. Meeting with Kazuha a few days later, Haruka informs Kazuha that her father had been supporting Akira financially in secret. When Kazuha witnesses her father praising Akira, she realizes that she had misjudged him. She is able to move past her guilt-ridden emotions towards Akira and pursue a whole-hearted romantic relationship with Haruka.

The introduction to this episode replays the event where Haruka overhears Akira's and Kazuha's conversation; but this time, instead of walking away misled, Haruka learns the facts of the relationship and the story has an alternative ending. Haruka notices that Akira has been acting strange lately, and decides to talk to her alone at school the next morning. He sees the sad look on Akira's face and becomes worried, and wants to help. Is a new romantic relationship in bloom?

Haruka suggests a DNA test to resolve the paternity issues surrounding Akira and Kazuha;and Kazuha agrees. Haruka informs Akira that the DNA results will be out in five days, but Akira disappears on the fifth day. Haruka finds her at the train station, where they talk until the evening. Kazuha's mother appears, and divulges the events as they unfolded many years ago. Akira then reads the DNA results,which confirm that Akira is indeed born of a different mother.