The story is set in an alternative timeline where the world is split into three superpowers, Britannia, the Chinese Federation, and the European Union (previously known as the Euro-Universe). The story takes place after the Holy Britannian Empire's conquest on Japan in August 10, 2010 a.t.b. (1955 A.D.), 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 Lamperouge is an exiled Britannian prince who was sent as a bargaining tool to Japan, along with his sister Nunnally Lamperouge, by his father, Emperor Charles zi Britannia, after his mother, Marianne vi Britannia, was killed. When Marianne was murdered, Nunnally was set up as a false witness. Because of this, her sight and her ability to walk were taken from her. 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 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 (??? giasu?). This power, also known as the Power of the King (??? ? 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, with 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 his younger sister, Nunnally, can live happily. In the process, Lelouch becomes the leader of the resistance movement known as The Black Knights under his alter ego Zero, as he gains popularity and support among the Japanese on his way towards rebellion.