During adolescence, few pains rival the sting of unreciprocated affection. That is exactly what Natsuo Fujii experiences, as he becomes infatuated with the perpetually upbeat teacher Hina at his school. Crestfallen by this unattainable longing, he jokes with friends and shows up at a social gathering. There he encounters Rui, a girl whose boredom mirrors his own. Sharing their mutual awkwardness, Rui invites Natsuo to her home and proposes they sleep together, hoping the act will convince her peers that she is no longer naïve. Though still tormented by his feelings for Hina, Natsuo reluctantly consents.
Both unfulfilled by their inaugural encounter, they decide to go separate ways as strangers. Yet before he can fully digest the event, Natsuo"s father delivers a startling revelation: he will remarry, and his new spouse Tsukiko Tachibana is arriving soon to meet him. To Natsuo"s astonishment, her daughters—and consequently his new sisters—are Hina and Rui Tachibana, the woman he loves and the girl with whom he shared his first night. Now Natsuo must reconcile his emotions toward these step-siblings as the shadows of love deepen. He finds himself torn between familial duty and forbidden desire, questioning whether affection can ever be pure when it is entangled with blood ties. The complexity of his heart grows, and he must navigate a path fraught with ethical dilemmas and emotional turbulence.
