Disinterested scholar Mizuto Irido and shy geek Yume Ayai appeared to be a perfect pair, united by their shared passion for books. Yet over time their differences swelled and they drifted apart after graduating from middle school. Fate, however, brought them back together as step-siblings.
A new rivalry sparks between the ex‑couple, each refusing to admit the other as the elder sibling. To settle it, they set a rule: whoever oversteps the limits of sibling etiquette loses; the victor earns the title of older brother or sister and may make a wish. Now sharing a home, their old memories seep into their behavior—perhaps reigniting feelings that were never truly gone. Their shared past of late‑night reading sessions and quiet library corners now echoes in their daily interactions, turning simple gestures into charged moments. The tension between them grows with each subtle challenge, and even the smallest misstep could tip the balance of sibling hierarchy.
