Lets try to Translate this summary into something more English based on my knowledge of Japanese and MTL tendencies.
Airi Uemura has been in a relationship for close to 10 years ever since she was in High school. A former delinquent, she'd rather fuck than be fucked, so she's having trouble in bed with her older boyfriend Shingo Makihara, as they both want to be on top; despite this, both are thinking of marriage someday. During an indoor date thinking no one was around, Shingo approached her and they fucked again, but after finishing they bump into Shingo's little brother, Shinji who left work early due to an injury. Airi bumping into Shinji again the day after, asked him about that day, and became certain that he had seen them. Shocked and excited by the prospect of being seen, she takes him into her room, using him as a toy to satisfy her desires.