Nao's obsession with her brother stops being cute the moment she realizes he might not actually be her brother. The show pivots from standard little-sister trope mockery into something genuinely unsettling about identity and belonging. It's a comedy that suddenly demands you care whether the person trying to seduce someone is even allowed to want them.

Nao Takanashi is completely devoted to her older brother Shuusuke, and she's not shy about it. The second-year middle schooler flirts with him every day, hoping he will finally see her as a woman instead of a little sister, and he often ends up going along with her teasing. While clearing his porn collection out of his room, she comes across an album of his childhood photos, and she isn't in a single one. Learning the truth about their family only makes her more affectionate, but her hopes of keeping him to herself are soon under threat. Iroha Tsuchiura, a childhood friend Shuusuke hasn't seen in ten years, also turns up and throws herself at him, leaving him caught between the two girls in an ecchi romantic comedy.