Yuriko Shio becomes my new mother after my father's remarriage. Around that time, I entered college outside the prefecture and began living alone in a boarding house, barely exchanging words with Yuriko. Returning home for winter break, we decided to host a Christmas party together with my parents. After drinking, we both fell asleep under the kotatsu. I accidentally kissed the defenseless Yuriko's lips, and in that instant, my entire body was overwhelmed by a melting pleasure. On a quiet night, a bittersweet yet pure romance begins.