Yuuto follows Mikoto, a childhood friend and high school girl who is one year older, to enter the same high school. When she goes home with Mikoto for the first time in a year, it turns out that Mikoto only sees Yuuto as her younger brother. Yuuto is a little dizzy about the difficult road ahead, but an alien "onee-san" in a very erotic outfit appears in front of the two...!? "One-Shota" space opera opens here!! (Source: Kodansha, translated)