The story is set in an alternative timeline where the world has become split into three superpowers: the Holy Britannian Empire (the Americas; also called Britannia), the Chinese Federation (Asia), and the European Union (Europe and Africa; previously known as the Euro-Universe. Also known as Europa United in Akito the Exiled). The story takes place after the Holy Britannian Empire's conquest on Japan on August 10, 2010 a.t.b., by means of Britannia's newest weapon, the "Autonomous Armored Knight", or Knightmare Frame. In turn, Britannia effectively strips Japan and its citizens of all rights and freedoms and renames the country Area 11 with its citizens referred to as Elevens.
Lelouch vi Britannia is an exiled Britannian prince who was sent as a bargaining tool to Japan, along with his sister Nunnally vi Britannia, by his father, Emperor Charles zi Britannia, after his mother, Marianne vi Britannia, was killed. Nunnally witnessed the murder of her mother Marianne, which caused her to lose both her sight and ability to walk. This makes it difficult for Lelouch because he must take care of her while on the run in Japan during the war. After the war in the ruins of a Japanese city he then vows to his Japanese friend Suzaku Kururugi that he will one day obliterate Britannia.
Seven years later, Lelouch, now a popular yet withdrawn student at Ashford Academy by the name of Lelouch Lamperouge, gets caught up in a terrorist attack and finds a girl called C.C. (C2), who saves Lelouch's life from the Britannian Royal Guard, by making a contract with him that grants Lelouch a power known as Geass (ギアス giasu?). This power, also known as the "Power of Kings" (王の力 Ō no Chikara?), allows him to command anyone to do whatever he wants, whether bending their will to live, fight, or die on his behalf –though only once and only through direct eye contact. Lelouch decides to put his Geass to use and find the person who killed his mother, destroy the Britannian Empire, and to create a better world where Nunnally can live happily. In the process, Lelouch becomes Zero, a masked vigilante and the leader of the resistance movement known as The Black Knights, gaining popularity and support among the Japanese on his way towards the rebellion of Britannia. However, this does not come without a cost. Caught up in a conflict where he does not know the full extent of his powers, Lelouch will have to battle Suzaku, a resistance member named Kallen Stadtfeld, the strongest army in the world, his own half-siblings, and many others in a battle that will forever change the world.