"I can only ask my mother... right? Isn't it okay? I'm telling you!!" After being scolded by his girlfriend for being bad at sex, Koji begs his mother Yumiko to be his practice partner. "Okay... but only once in return." Yumiko refuses several times, but she gives in to Koji's enthusiasm and accepts. "No, I just can't... please stop!" She comes to her senses and resists the forbidden act, but his reckless caresses gradually bring changes to Yumiko's body, and she reaches climax many times...