Akira, a college student who is not popular with girls, gets a new step-sister, Natsu, after his parents remarry. Natsu is foul-mouthed, but in front of his parents she is a serious and cute girl. She thoroughly teases her brother, saying things like, "You're not popular with girls, are you?" and "Women will never feel good with your small dick." Akira loses his temper with this attitude and attacks Natsu.