Code Geass is a 50 episode long anime that ran from 2006 to 2008. It has two parts, each 25 episodes long. Until today, it is considered a critical acclaimed anime.
The story of Code Geass is set in a fictional world in the year 2018 in which three superpowers control the world: Britannia (America), the Chinese Federation, and the European Union.
Go back eight years earlier: The ten year old boy Lelouch vi Britannia is one of the many, many, many children of the emperor of Britannia (that jerk had a lot of wives). One day, Lelouch's mother gets mysteriously killed in the middle of the night while his three year old younger little sister Nunnally is shot in the legs. She ultimately looses the ability to walk and becomes blind as a result of the traumatic experience she had. Upon asking his father why he couldn't set up the night guards to protect Lelouch's mother, his father exiles Lelouch and his sister for their disobedience (crazy that emperor). Lelouch is sent to live with the Japanese govenor and quickly befriends a Japanese boy named Suzaku Kururugi. One day, Britannia, hungry for further expansion of their territory, invades Japan, forcing the govenor to surrender and commit suicide. Japan is now referred to as Area 11 and is a colony of Britannia. Lelouch and his sister separate their ways from Suzaku and don't get to see him again until many yrs later.
Eight Years Later: Lelouch and his sister now live in the Ashford Academy for students from Britannia in Japan, hiding their true royal identity from everyone. Everything is perfectly fine, but Lelouch is unsatisfied with his life as he wishes to take revenge on his father who left him behind. One day, Lelouch sneaks out of the classroom (like always) and gets caught up in the middle of a car chasing. When the police officers chase a truck, the truck crashes. Lelouch climbs in to see if everyone in there is ok. Unexpectedly, he finds a greehaired girl imprisoned inside a metallic ball in the back of the truck. Her name is C.C (say C two). When Lelouch is discovered by the Britannian Army, he finds out that Suzaku is also a Britannian soldier (you might ask why a Japanese boy would join the enemies, but you'll see why in the future). When Suzaku gets shot while protecting Lelouch (don't worry, he didn't die), Lelouch tries to run away with the girl. C.C. then grants him the power of Geass while performing a contract with her. Geass is a supernatural power that whomever gains it from C.C. can perform something miraculously. In Lelouch's case, it is the power to order other people around when he looks at someone directly in the eye.
That's basically the setup. Lelouch, you see, is your anti-hero. He's smart, well-worded, handsome, but physically not the strongest. With C.C and the power of Geass he assumes the identity of Zero and becomes the anonymous leader of some Japanese revolutionists who battle Britannia. Lelouch's only weakness is Nunnally for whom he wishes to create a better world for so that even she as a crippled girl can live in it. Suzuku is the deutagonist who will constantly try to chase Zero, his main rival and ironically also his best friend, unbeknownst to him, though.
One last thing is that Code Geass is considered a mecha anime b/c fights usually occur in those so called Knightmare Frames.