Sakura Momonogi is getting into risque outdoor sex that she hasn't experienced before... In a family restaurant, a park, the school, a karaoke box, at the beach...
A schoolgirl who looks like a mature woman is asked to be a girlfriend of her childhood friend, but that ship has already sailed... This sort of NTR (cuckoldry) doujinshi.
I (Satoru) had a sister (Serina) who went to the same school as me. I was pretty short, and very timid. My beautiful sister would always be around to protect me...