The story is set in an alternative timeline where the world has become spilt into three superpowers.

Lelouch vi Britannia or lelouch lamperouge is an exiled britannian prince who was sent as a bargaining tool to japan, along with his sister nunnally via britannia, by his father, emperor Charles vi Britannia, after his mother, Marianne vi Britannia, was killed

Nunally witnessed the murder of her mother Marianne, which caused her to lose her sight and the ability to walk. This makes it difficult for lelouch because he must take care of her while on the run in Japan on the war