結城友奈は勇者である 花結いのきらめき

One day during their club activities, the six members of the Sanshuu Middle School Hero Club are engulfed in a mysterious light. When they return to consciousness, the world has forestized and is invaded by Vertexes. For some reason, the Transformation App which should have become unnecessary has returned to them. After they repel the Vertex despite their confusion, a girl who calls herself Uesato Hinata appears before Yuuna and the others.

According to Hinata, they're in a "special world" inside the Shinju. The Shinju is an aggregation of land gods, but before long this strong union will break apart through rebellion and the inside will be devastated. She tells them that the Vertexes from before were imitations created by the rebelling gods. If the land gods lose their unity, the Shinju will lose a great amount of its power.