In the past, there was a goddess whom extended a helping hand to a world which was about to perish.
That goddess, along with the four knights who served her, healed the ruined land using "Grace"- a power that rejuvenates worn down life forces- and saved the world.
From then on, since the goddess' consciousness went into a slumber, she was succeeded by one who is to become a representation of her divine spirit, known as a "Reine", and four human knights that were chosen by the world. As the world continued to enjoy the goddess' favor, it continued to develop and advance.
However, at a certain point, even after coming into the world for generations, a Reine was not born. As a result, the supplies of Grace stagnated, and the world was once again threatened to be consumed by darkness.