The story begins after mysterious creatures named Archons suddenly appear in the world. Humanity faces the threat of annihilation from these creatures. Only the Chalice, the source of humanity, can defeat the powerful Archons. So, the humans assemble an Archon-slaying unit from fighters who have the ability to bring forth the Chalice to fight. The ones chosen to fight the Archons with the Chalice as their weapon are called Goulart Knights.

(Source: Anime News Network)