Takasora Hoshi lives a cramped and unremarkable life in a small apartment with her mother. That mundane existence is upended during one hot summer when a classmate named Umi Mizuike and her mother come to live with them temporarily. From the beginning, Takasora finds herself irritated by Umi’s odd behavior, yet she is also strangely drawn to the other girl’s quiet beauty. The two teenagers agree to keep their distance and not interfere in each other's lives, but Takasora cannot shake her curiosity about where Umi slips off to every night, returning in the morning smelling of perfume.

As Takasora’s initial annoyance blossoms into a genuine crush, she secretly follows Umi and discovers the devastating truth. Umi has been meeting a wealthy, charismatic older woman who calls herself Chiki. Having grown up in poverty and endured years of neglect and abuse, the vulnerable Umi has entered into a transactional sexual relationship with Chiki, who provides her with money, affection, and a twisted sense of security. Horrified yet unable to look away, Takasora watches as Chiki kisses Umi, only to be spotted herself. Rather than reacting with anger, the manipulative Chiki invites the heartbroken Takasora to join them, using Takasora’s obvious feelings for Umi as leverage.

The situation spirals as Takasora is pulled deeper into Chiki’s web. Chiki, a predator obsessed with high school girls, delights in psychological games, forcing the two younger girls into uncomfortable situations and teasing them with the promise of intimacy. The story reaches a shocking climax when Chiki reveals her true identity. She is not merely an anonymous benefactor but Shiho, Umi’s long-lost older half-sister. This revelation, which should be horrifying, only deepens Umi’s obsessive devotion to her sister, who has been grooming her all along.

The final volume follows the aftermath of this revelation. Umi, far from being repulsed, embraces the incestuous relationship, while Takasora desperately tries to pull her back. In a futile attempt to compete, Takasora allows Umi to use her body, but Umi bluntly confesses that her heart only races for Shiho. Umi and Shiho eventually run away together to a seaside town. When Takasora visits, she finds Umi utterly transformed, her hair, speech, and personality reshaped to fit Shiho’s desires. Umi reveals her plan to end her life after graduating high school, believing Shiho will discard her when she ages out of her preferences. Defeated and unable to save the girl she loves, Takasora returns home. In a dark, cyclical epilogue, Takasora is implied to have followed the same tragic path as Shiho, becoming an older woman preying on vulnerable young girls. Her first love ends not with a kiss, but with a complete and devastating destruction of innocence.