Nao and I aren't actually siblings. When my father remarried when I was a student, Nao was my stepdaughter. After my parents died in an accident, Nao, who was much older than me, became like a parent to me. Although it was lonely living with just the two of us, I was also happy because I had a crush on Nao. One day, I heard my best friend Yuzuru whispering with Nao, and feeling jealous, I approached him, but she skillfully evaded the question. Then, after a certain point, Nao started coming home late...