After the sudden death of their parents, Haruka and his twin sister Sora move to the house formerly inhabited by their late grandfather, 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, refusing to go to school even as Haruka starts attending. Realizing that Haruka is growing more and more distant and slipping from her grasp, Sora goes to Haruka's room at night and appeals to Haruka as she removes her clothes...

Sora asks Haru to take her measurements so she can have a uniform made, and 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 who forgot her swimsuit, and so Akira takes her to dry off. Haruka finds Kazuha's phone and goes to return it, only to overhear a sensual-sounding conversation in which Kazuha addresses Akira as "Onee-chan" (big sister).

When Kazuha realizes that Haruka has overheard their conversation, she chases him down 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. To help lift Kazuha's spirits, Haruka invites her out on a date, and they subsequently realize and admit their rapidly-progressing feelings towards each other. Meanwhile, Akira continues to try her best on her own, but collapses, apparently due to exhaustion.

On their way home from school, Haruka and Kazuha are interrupted by a frantic Yahiro, who is concerned over the non-appearance of the always-responsible Akira as usual. They discover Akira at her home, collapsed due to exhaustion, and Kazuha proceeds to blame herself and her infatuation with Haruka for not noticing Akira's condition.

Haruka is standing near the girls locker room, overhearing Akira's and Kazuha's conversation. 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 Akira find her mother's pendant (after remembering her losing it as a child). They go in search for the pendant by a tree they would play by as kids, only to Haruka's horror that the tree had been removed by a landslide. He attempts at digging around for it, but without any luck. The two go to Akira's to take a bath, and have a romantic moment together. From there, Haruka goes to the festival, and, after seeing Akira cry while dancing the sacred dance, rushes off to see what had happened.

Haruka enters Akira's room and stops her from burning something, which is her mother's missing journal, where it is written that Kazuha and Akira has a chance of mistaken identity. With the journal, he suggests to Yahiro and Kazuha a DNA test to prove things once and for all.

By now Sora is showing her hatred of Nao in the open, even in front of Ryohei and Nao herself. One afternoon Sora, attracted by the smell of curry cooking in the kitchen, catches Haruka and Nao making love in the living room. Sora throws Nao out of the house, warning her not to see Haruka ever again, since it is the second time she catches the two doing the "dirty deed."

Haruka is trying to start a new life over with Nao, but Sora is always at the back of his mind, since, lately, she seems not to accept Nao, and that she seems to be teasing him. Haruka suggests a date with Nao, but cannot get to tell Sora about it, even disguising it as if he will be spending time with Ryouhei. During the date, though, Nao notices that Sora is never far from Haruka’s mind, him buying things for his sister rather than for her, and eventually spoiling their date. They plan to make croquette curry upon arriving home, but is canceled when Haruka finds Sora not feeling well in her room after having a bad dream. The next day Nao notices that Haruka tries to hide his date with her from Sora, but she just went along. One night, Haruka catches Sora masturbating in her room while mentioning his name, much to his dismay—she is enamored with him ever since that one summer day in their childhood when Haruka kissed Sora while playing.

Kozue runs away in shock at the sight of the twins having Intercourse, while Nao calmly left the scene, but Sora wouldn't even care. Haruka thinks of ditching school, but she tells him to think little of it. Kozue tells him the next day that she'll never talk to him again, while Nao effectively ends her relationship with him after his apology. That night Haruka fights with Sora when he tells her that they stop doing it, even inadvertently hitting her. Things get complicated when they find out that their parents left no money for the two, and in order to survive they have to be taken under custody of their relatives, but this means them living apart.