Owarinaki Natsu, Towa Naru Shirabe

In Kyushu, there lies the small village of Suno. In that small village, four childhood friends, Touma Takumi, Ogami Ritsuko, Amamiya Kanon, and Takanashi Mio comprise a small music club at the village school. One day, Takumi happens to see and hear the violin performance of Orihara Ai, a girl who recently moved to the village and transferred into the school. This performance later eventually causes Ritsuko to resolve to recruit Ai into their music club no matter what.

However, one day, one of the childhood friends is separated from the other three, but before doing so passes on her role of "Supervisor" to Takumi. Later, they learn that the school they all attend is going to be closed down in one month due to a government-ordered evacuation of the village. Everyone resolves to make their last month together as memorable and enjoyable as possible before the school closes down...

[Edited from the Muv-Luv Wiki]