When his wife refuses to let him have sex because she's tired from raising their child, Kazuki comes to a box health club, where he bumps into Kana, a beautiful mom friend from his son's class. It's awkward, but she gently helps him out, saying, "It's our little secret, okay?" He feels guilty, but the sense of immorality and excitement of his wife's mom friend helping him out is even more overwhelming, so he returns to the box health club.