JUR-184 Mariko Sada, a married woman, secretly educates her sister's boyfriend with creampies, an affair with her husband abroad.

A man becomes the boyfriend of his senior at work, Saeko, but she won't have sex with him because he's a virgin, so he tries a dating app. He meets Saeko's older sister, Mariko, who is frustrated because her husband is abroad, and they start a secret friends-with-benefits relationship, with him getting creampies. The sex is so good that his desires escalate, even sneaking around at home.