Mizuto and Yume are a former couple who enjoyed a relationship in junior high school, but became more and more irritated at each other, and used their graduation as an opportunity to break up. But on the day before entering high school, the two reunite in the most unlikely way: with their parents announcing a marriage, with Mizuto and Yume now becoming step-siblings. Wanting to prioritize their parents' relationship, the two promise to not fool around with each other, establishing a rule where "the first one to get aroused" loses.