The story takes place in the era of the gods, year 300. Yuuna Yuuki lives an ordinary life as a second year middle school student, but she's also a member of the "Hero Club," where club activities involve dealing with a mysterious being called "Vertex."

Aired along side Yuuki Yuuna wa Yuusha de Aru: Washio Sumi no Shou Movie 3 - Yakusoku.

(Source: Anime News Network)