The person who sneaked into my bed that night was my girlfriend’s little sister. “If we meet again, then let’s be… lovers.” It was the promise I made to my childhood friend, Mahiru Mitsuha, on a distant day. In the spring of my second year of high school, we had a fateful reunion. We conveyed our unchanging feelings to each other, and we became a couple. It was supposed to be a happy life together with Mahiru, who left her parents and came to live in my house. However, someone else was with her— It was Saya, Mahiru’s younger sister. One night, Saya sneaked into my bedroom and gave me a kiss. “Shh, don’t make a sound. And keep this a secret from Onee-chan, okay?” From that moment on, the life with the three of us and our secrets began. Until the day I crossed the line with my girlfriend’s little sister.