There is a small town somewhere in Japan, which is surrounded by nature. In this town, there is a large and gorgeous school, Otobashi Gakuen, which was founded to make the students familiar with nature. 

This story starts off with Kurusugawa Himeko. She is a normal student who goes to Otobashi Gakuen. She leads bright and enjoyable school days with her two friends, Himekawa Chikane and Oogami Souma. 
Chikane is the only daughter of a distinguished family. She excels at both academics and sports. She is also kind and beautiful, making her very popular among the other students. 
Oogami Souma is Himeko's childhood friend. He has problems expressing his emotions directly, but he is very sincere. 

On Himeko's 16th birthday, a dark moon blocks out the sun, and the Orochi come forth to destroy the world. But why are they targetting Himeko...? 
Based on the manga by Kaishaku