There's a secret that has never been told to commoners. Since ancient times, humans have always been protected by chevaliers. Those who were born with supernatural powers called vistal that allowed them to combine elements with technology and pass judgment on malus, an evil demon who wants to rip the soul of humans. There are four basic elements: Ignis (fire), Aqua (water), Terra (earth) and Ventus (air). 

Chevalier act based on their persona. A character with their responsibility. There are four personas for chevaliers to act based on: book (the one to record), pencil (the one to write), eraser (the one to correct) and ruler (the one to measure). They will be given this character by their superior. Only those who are born with a chevalier bloodline will have at least the power to control one type of element and possess one type of persona when the time comes.

Their sole objective is to fight a malus who's also living on earth and intending to secure the earth for themselves. The malus is always devouring humans with their devilish thoughts. 

Chevaliers will pass judgement on the malus with their gifted powers. But there is one condition, they can only use their power at 20 years old. They cannot utilize it before or after the age of 20 years old. 

Being chevaliers is a secret that can't be told to a normal human. It is a law for chevaliers to hide their identities from normal people. As a great burden not meant to be shared with unprepared souls. 

In 2018, Hikari Yama, a 16 years old chevalier boy, proceeded with his ancestor's mission of keeping world peace. Living his life as someone with a chevalier bloodline. But far different from the chevalier's mission, he wants to be free from the power he has.