A couple of parents married with children. The father has a son and the mother has a daughter, so they are stepbrother and step-sister. One day, step-sister Mio's underwear goes missing. When she happens to peek into her step-brother's room, she sees him masturbating with the lost underwear. They start fighting to get the underwear back, and the momentum leads to rape...and then, he accidentally inserts it into her anus...