Yumi is an education-oriented mother who is eager to get her son Yosuke into the same prestigious school as her husband. She is flawless, but once a month, on her dangerous day, her body becomes very sensitive and she can't stop being aroused. Yosuke, who secretly knows about this, pretends it's an accident and shows her masturbation on the dangerous day to vent his frustration... Yumi, who hasn't had sex in a long time because her husband is away on a business trip, doesn't care that it's her son, but she devours his erect cock and swings her hips lasciviously as her instincts dictate...