神世紀300年。主人公・結城友奈は、ごく普通の平和な生活を送っている中学2年生の女の子。
彼女は、「みんなのためになることを勇んで行う」という活動趣旨の「勇者部」という部活動に所属し、校内校外を問わず様々な活動に励んでいた。
しかし彼女のそんな日常は、「バーテックス」と呼ばれる怪物の襲来によって、いったん終わってしまう。
Yuuki Yuuna wa Yuusha de Aru: What does it mean to be a hero? Yuuki Yuuna thinks she knows. As part of the Hero Club in her middle school, Yuuki always does her best, wants to help others, and strives to bring smiles to people's faces, no matter who they are.
But Yuuki is about to become a much bigger hero. Mysterious and powerful forces are approaching and threatening to destroy the world Yuuki knows and loves, and it's up to Yuuki and the Hero Club to stop them. Together with her companions, Yuuki uses a special phone app to transform into a magical girl and battle against strange beings called vertices.
In between studying and putting on shows for kids, Yuuki has to fight for the very existence of her world. Can five middle school girls really stop the vertices from entering their home world and destroying it?
