Mizuto Irido and Yume Ayai shared a bond as bookworms but drifted apart after middle school due to their differences. However, destiny brings them together once again as step-siblings. A competition ensues as Mizuto and Yume fight to assert their role as the elder sibling. They devise a rule to determine the winner – whoever crosses the line of siblinghood will concede while the champion earns the privilege of being called the older sibling and making a demand. Yet living together resurrects old recollections and could reignite suppressed sentiments.