Fraternal twins Yori and Iku Yuki share an unusually close bond from childhood, but as they grow older, Yori begins to distance himself from his sister. The reason is a secret he can no longer hide: he has fallen in love with Iku. Ashamed of his feelings, he tries to suppress them by dating Iku’s friend, Tomoka Kusunoki, and plans to transfer to a high school far away from home. Iku, confused by her brother’s sudden coldness and eager to keep him close, finds herself drawn into his emotional turmoil. When Yori finally confesses the truth, she is shocked but reluctantly kisses him, afraid that rejecting him might drive him away for good.

Set against the backdrop of their shared home, school, and the ordinary spaces of Japanese adolescence, the story follows the twins as they navigate a secret romantic and physical relationship hidden from their parents and peers. Iku struggles with guilt and her own growing feelings, while Yori remains torn between elation and self-loathing. He breaks off his relationship with Tomoka, but maintains his plan to leave, believing that staying would prevent them from ever acting as a normal couple. With the help of Yori’s friend Haruka Yano, who knows their secret and secretly falls for Iku himself, they continue to meet in secret after Yori transfers to a new school.

The narrative takes a dramatic turn when a family friend, Dr. Yuugo Mori, enters their lives. Yori begins to suspect that Dr. Yuugo is his biological father, and a DNA test confirms that Yori was fathered by Yuugo while Iku remains the daughter of their father Shunpei. Their mother admits to the affair on her wedding night, revealing that the twins are actually half-siblings through heteropaternal superfecundation. Learning they are not biologically related in the usual sense, Yori confesses their relationship to his mother, who insists they are still siblings. Facing the threat of being separated, Yori and Iku run away from home with Haruka’s help. Eventually, Yori makes the painful decision to end their relationship and reconcile with their parents, then leaves for a place where he can never be found, hoping their feelings will fade with time.

Ten years later, Iku works as a sales representative, having rejected several marriage proposals from Haruka. While on a business trip to England, she encounters Yori again. When they recognize each other, they embrace, and Yori declares that he is still in love with her. The film closes on this ambiguous reunion, leaving the ultimate fate of their relationship unresolved.