Yuki Yuna is a Hero Season 1 Episode 1

A Maiden's True Heart

Yuna Yuki, her best friend Mimori Togo, and sisters Fu and Itsuki Inubozaki, are all members of the Sanshu Middle School Hero Club, which is dedicated to helping those that request it via their website. During class one day, upon each receiving a strange 'forestize warning' message on their phones, the four girls find time has suddenly stopped around them before they are enveloped by a rainbow-colored light, soon finding themselves in a strange colorful forest. Fu, who had told the others to download a special app on their phones when they first joined the Hero Club, reveals that their club has been chosen by Shinju, the divine tree, to fight against mysterious enemies known as Vertex, whose goal is to destroy Shinju, killing the world as a result. When one of these Vertex appears and starts attacking, Fu tells Yuna and Mimori to escape while she and Itsuki use their phones to transform into heroes to fight against the Vertex. Determined to protect Mimori, Yuna awakens her potential as a hero and transforms herself, using her newfound strength to fight against the Vertex.