My beautiful sister and I, who should never be able to touch her, are stepchildren who are not related by blood... On a summer day, when the two of them, who have lost their bondage as siblings, fall in love with forbidden creampie affairs.

On a summer day when the sound of cicadas was loud, I had returned to my parents' house for the first time in a while, on my mother's 17th anniversary. The fact that I always went home every year was the presence of my older sister, Riho. She is the older sister I admired, who has always looked after me, replacing my mother, who passed away early on. My sister and I have grown up and are married, but I still have special feelings for my sister that are different from my siblings. Then, the night after the memorial service was over, my father called me out and I was told that I was not a real sibling with my sister Riho...