Mizuto and Yume used to date, but now they've suddenly become stepsiblings. Both of them agree that they don't want to do anything that'd disappoint their newly remarried parents. However, neither of them can accept the other as being the eldest sibling in the relationship. So they both agree to settle the matter with a game. Whoever does something in conflict with what people might consider normal sibling behavior, loses. Making them the younger sibling.

After entering high school, Yume was making an effort to change her personality and looks in an attempt to reinvent herself as a model student. It was all so far so good, but there was one thing she couldn't change about herself. She was never very athletic. On the day of the physical fitness test and body measurement check at school, a melancholy Yume is approached by Minami Akatsuki, a girl from the same class. Meanwhile, Mizuto is as disinterested as ever...

It's Mother's Day, so Yume and Mizuto go out to buy a gift together. Their mother, Yuni, is overcome with happiness to receive a gift from the two of them. Seeing how happy it made her, Mizuto wonders if it'd be nice to give their parents some time to be newlyweds together. However, there isn't much they can do for them on a high schooler's budget. So Mizuto suggests that he and Yume spend the night somewhere instead.