My innocent angel had become so innocent that she was like a fallen angel.

Tachibana Momo, who started living alone in high school and just became a working adult this year. And Soshiya Yukie, a high school student who moved into the neighboring room to follow Tachibana. When Tachibana was in middle school and Soshiya was in elementary school, Tachibana kept saying "marry me" to the incredibly cute Soshiya. As a result, the innocent and pure Soshiya began to seriously consider marriage with Tachibana. And now, having reached an age where marriage is possible, Soshiya has appeared before Tachibana. A story of beginnings between an ordinary working adult and a slightly yandere-like high school girl.