On August 10, 2010, a.t.b., the Holy Empire of Britannia overpowered Japanese forces and conquered the country with their robotic weapons, the Knightmare Frames, in less than a month. In the aftermath of Britannia's invasion, Japan lost its freedom and rights and was renamed Area 11. The Japanese people, renamed as "Elevens", were forced to survive in ghettos, while Britannians lived in first-class settlements. Rebel elements persisted, however, as pockets of Japanese organizations who struggled against the Empire for the independence of Japan.

After his father, the Emperor of Britannia, did nothing to pursue the terrorists who murdered his mother and crippled his sister, the young prince named Lelouch vowed to destroy Britannia. Seven years later, living in Area 11, he encounters a mysterious girl who gives him the power of Geass. With it, he finally has the power that he needs to defeat Britannia and fulfill his two wishes: to seek revenge for his mother and to construct a world in which his beloved sister can live happily.