As his wife nears childbirth, her mother Ikumi comes to Tokyo to help. Her presence eases the pregnant wife's burden, but the husband has also been forbidden to masturbate and struggles to hide his erection whenever he smells his mother-in-law's scent. One day his restraint snaps and he spies on Ikumi changing after her bath. A noise exposes him, but instead of condemning him, she senses there must be a reason—and makes an unexpectedly obscene proposal that leads to repeated breeding sex...