Mizuto Irido, a lackadaisical geek, and Yume Ayai, an introverted bookworm, once a perfect literary pair, drifted apart post middle school due to their evolving differences. Ironically, they reunite as step-siblings, sparking a rivalry over seniority, refusing to concede to one another. To settle the dispute, they establish a rule: the sibling who violates traditional norms forfeits, granting the victor the title of elder and a wish. Living together reignites buried emotions, hinting at a rekindling of unresolved sentiments.