Yuuki Yuuna Is a Hero

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.

With the Vertex able to regenerate its wounds, Fu has Yuna and Itsuki perform a sealing ritual in order to expose its core. Working together, Fu and Yuna manage to destroy the core, defeating the Vertex and returning everyone to the real world. The next day, Fu explains that she was tasked by her organisation, the Taisha, to grant hero candidates the power to fight against the twelve Vertex that threaten Shinju. Mimori, frustrated that she couldn't help everyone during the battle, directs her anger at Fu for keeping things a secret from everyone, but Yuna manages to cheer up a little. The girls are soon brought into another battle as three more Vertex emerge, using combined attacks to keep them from performing the sealing ritual. When the Vertex start attacking Yuna, Mimori becomes determined to protect her and awakens her hero form, using long range attacks to provide support, and together they manage to defeat all three Vertex.

A month following their last encounter, the girls prepare to face off against the fifth Vertex, only for another hero named Karin Miyoshi to suddenly appear and defeat it single-handedly. The next day, Karin transfers into Yuna and Mimori's class, claiming herself to be a fully trained hero sent to supervise the newbies. She reluctantly joins the Hero Club, though is stubborn about becoming friends with the others. That weekend, as the girls plan to meet somewhere, Karin skips out on it, feeling she should not be led astray from her duty as a hero. Later that night, the girls show up at Karin's apartment, revealing they had planned to surprise her with a birthday party. At the end of the day, Karin is starting to get along with them.

Itsuki, who gets nervous when singing in front of others, is anxious about an upcoming test, so the Hero Club decide to go to karaoke to help boost her confidence. Meanwhile, Fu is unnerved by a message from Taisha telling her to expect the worst. As she continues to struggle with her nerves, Itsuki thinks about the burden placed on Fu, who has been looking after her ever since their parents died. Later, as Fu laments getting Itsuki involved with the Vertex, Itsuki assures her that she feels happy being able to fight alongside her. On the day of her test, Itsuki finds a letter from everyone encouraging her to do her best, allowing her to sing with confidence and giving her a goal of becoming a singer. Soon afterwards, as Fu contemplates with worst case scenario, another forestization appears.

Facing up against all seven of the remaining Vertex, the girls start to fight against the first one, only to be caught in a trap by another, who uses a loud bell to disable them. Itsuki manages to stop the bell, but then three Vertexes merge to form a single more powerful Vertex, proving resilient to their attacks. Not willing to give up, Fu and Mimori use their trump card, the Mankai form, to defeat the combined Vertex, while Itsuki uses her Mankai form to stop a smaller, faster Vertex from reaching Shinju. Fu then risks herself to buy time for the others to seal the remaining Vertex, whose core is discovered to be a massive one positioned in the stratosphere. Yuna and Mimori head to space together in order to fight against the core, using their Mankai forms to destroy it. Following the battle, Karin is relieved to find everyone alive and well and reports their victory to Taisha.

After receiving checkups at the hospital following their battle against the Vertex, Fu is revealed to have lost some vision in her left eye while Itsuki has lost her voice, both of which they assume to be temporary symptoms caused by battle fatigue. However, upon discovering Yuna has also lost her sense of taste and that she herself has gone deaf in her left ear, Mimori fears that these symptoms could be a side-effect of the Mankai form and gets in touch with Fu about it. Meanwhile, Karin, who hasn't been affected since she didn't use the Mankai form, has been avoiding going to the clubroom, as she fears she no longer has any purpose now that the Vertex are gone. However, Yuna manages to find her and convince her that, even with no-one to fight, the Hero Club will still carry on like it always has. Touched by Yuna's friendship, Karin convinces Taisha to let her stay at Sanshu Middle School until graduation, while the girls make plans for summer break.