Taking place in the realm of deities during the year 300, the story follows a standard second-year middleschooler named Yuuna Yuuki, who also happens to be a member of the "Hero Club." The club's main purpose is to fight against a perplexing entity named "Vertex." The series aired back-to-back with the film Yuuki Yuuna wa Yuusha de Aru: Washio Sumi no Shou Movie 3 - Yakusoku.