When he comes back from his job, he feels strangely erotic about his daughters' bodies, which are still in the process of development. The daughter notices her father's erect penis and says, "We're father and daughter, but we can't do that!" and seems to want it. When his erect penis touches her defenseless ass, showing her panties, the father and daughter are both excited and can't contain themselves, so they cross the line and insert it without letting the other family members know.