Based on the light novel series Washio Sumi wa Yuusha de Aru written by Takahiro and illustrated by Bunbun.

In the year 298 from the era of the gods, elementary school girls Washio Sumi, Nogi Sonoko, and Minowa Gin are tasked with an important mission. They are to become heroes and fight Vertex, a mysterious enemy that is attacking Shinju-sama, the god tree that protects Shikoku, the only area in the world that is still habitable. What they don't know, is that this fight will cost them more than they could have ever imagined.

(Source: MU)

Note: It is a prequel of Yuuki Yuuna wa Yuusha de Aru later included as the first six episodes in Yuuki Yuuna wa Yuusha de Aru's sequel Yuuki Yuuna wa Yuusha de Aru: Yuusha no Shou.