It's not okay to do this with my brother... The main character, Mizakura, and his twin brother go on a 3-night, 4-day trip to an unexplored hot spring inn. Just when they thought they could just relax in a hot spring, the brothers, whose personalities don't match, suddenly start fighting over the younger sister they've always loved. The older brother Rei is calm and strategic, and the younger brother Yu is passionate and straightforward. Although they are different types, the rivalry between the sadistic brothers gradually escalates into one where they seek each other's bodies. What will Misakura choose in the end?