Kazami Yuuji is picked up by the police as a suspicious person after arriving in town. He is later released after a computerised search on his name reveals that he seems to have some connection with the Ministry of Defence's special department. He is picked up at the station by the school principal Tachibana Chizuru and brought to Mihama Academy, where his wish to have a "normal school life" is supposed to be fulfilled. The nature of the school is explained to him, there being only six students, including himself. He meets four of the other five students, all of whom seem in their own way to be rather strange.

One day while having fun with her classmates, Amane sprains her leg during PE. Mihama Academy has no resident nurse, so Yuuji, who learned first-aid at his previous school, treats her injury. While he tends to her, Amane confesses how she feels and says "Be my boyfriend." It`s so sudden Yuuji comes to the conclusion she`s teasing him. Amane says in response, "If you won`t be my boyfriend, then I`ll be your big sister!"