Elementary school students Sumi Washio, Sonoko Nogi, and Gin Minowa are all chosen as heroes to protect the Shinju from the Vertex. Thrust into their first battle against a water-based Vertex, the girls initially struggle to attack it, but soon manage to push it back with teamwork and guts. Following their narrow victory, Sumi works up the courage to ask Sonoko and Gin to be her friends. Half a month later, after the girls beat their second Vertex, they are sent on a training camp to build their teamwork. Later, Sumi and Sonoko follow Gin as she winds up helping a lot of strangers, when the third Vertex suddenly appears. As the girls manage to work together to defeat, Sumi laments that, contrary to her own beliefs, she was the one faltering behind everyone and swears to become stronger.

Given a break from their training, the girls take some time off, during which they take about their dreams and prepare a special presentation for the freshmen's orientation. Later, after returning from a school trip, the girls fight against two more Vertexes, only for Sumi and Sonoko to get heavily injured when a third one appears as well. Being the only one left able to move, Gin faces the Vertexes herself and manages to defeat them but ends up dying from her injuries, leaving Sumi and Sonoko devastated.

With Gin's death weighing heavily on their minds, Sumi and Sonoko are forced to fight another Vertex in the middle of Gin's funeral. Wanting to cheer Sonoko up, Sumi invites her to a festival where they reaffirm their friendship. Later, Sumi and Sonoko receive fairy partners as part of a new hero system, the details of which are only told to their parents. Dragged into another battle gainst three more Vertexes, the girls take advantage of their new abilities and use the power of Mankai to defeat two of them. However, Sumi then discovers she has lost the use of her legs while Sonoko has gone blind in one eye. Despite concern over these inflictions, the girls continue to use Mankai to stop the remaining Vertex from attacking the bridge. After managing to destroy the Vertex's main body, Sonoko follows its core through a barrier, where she witnesses the previously defeated Vertex being resurrected. Sonoko further discovers that her heart has stopped beating but is somehow still alive, while Sumi no longer possesses any memory of her as a result of her second Mankai. Realising that she is no longer capable of dying, Sonoko undergoes Mankai countless more times to protect Sumi from the ressurecting Vertex. Some time later, Sumi, now going under her original name of Mimori Togo, moves into a new home and is befriended by her new neighbor, Yuna Yuki.