Aki, an unpopular college student, gains a new stepsister, Natsu, due to his parents' remarriage. Natsu has a foul mouth but acts like a diligent and cute daughter in front of their parents. "You're not popular, right, big brother?" "No woman would ever feel good with your worthless dick," she relentlessly provokes him. Enraged by her attitude, Aki lunges at Natsu.
