On August 10, 2010, a.t.b., Japan was conquered by the Holy Empire of Britannia. Seven years later, teenage student Lelouch Lamperouge finds himself pulled into a battle between anti-government terrorists and the Britannian Army in the Shinjuku Ghetto. In the midst of the conflict, a mysterious, green-haired girl saves him from death by granting him "the power of the king", which allows him to compel others to do his bidding.

Using his newfound power, Lelouch hijacks an enemy Sutherland and leads the rebels in a counterattack against the Britannian army, forcing Viceroy Clovis la Britannia to call for the release of an experimental Knightmare Frame, the Lancelot, piloted by Suzaku Kururugi. The Lancelot proves to be far superior to the previous Knightmare Frame models and Lelouch is forced to change his plans. Just as the Britannian Army corners the terrorists, Prince Clovis orders an immediate cease-fire, as he is being held at gunpoint by Lelouch.

Lelouch confronts Clovis about the murder of his mother, Marianne vi Britannia, before killing him. When he returns to Ashford Academy, he discovers that Kallen Kōzuki, one of the terrorists he helped in Shinjuku, attends the same school. The limitations to his newfound power are also revealed to him when he attempts to question her then erase her memory, so he creates a cunning deception to ease her suspicions of his identity. When the death of Prince Clovis is announced in the media, it is Suzaku Kururugi who is accused of the murder.

The Purist faction within the Britannian Army scapegoats Suzaku for Clovis' death, planning to use him as an excuse to rid the army of Honorary Britannians. Lelouch concocts a plan to save his childhood friend, with some help from Kallen and Kaname Ohgi, leader of Kallen's resistance group. Announcing himself publicly as Zero, Lelouch declares it was he who killed Clovis, and uses his power on Jeremiah Gottwald to force him to assist in Suzaku's rescue. In private, Lelouch offers Suzaku the opportunity to turn against Britannia for falsely accusing him, but Suzaku refuses, wishing to change the Empire from within.

To Lelouch's surprise, C.C., the girl who gave him his power, is alive and now living in his residence. Suzaku, having been acquitted of all charges, accidentally meets Third Princess Euphemia li Britannia and ends up intervening in an internal Purist faction struggle. Meanwhile, Second Princess Cornelia li Britannia endeavors to take charge of Area 11 as Viceroy and bring Zero, now the Empire's number one enemy and most wanted man, to justice.

Suzaku transfers into Ashford Academy and faces the scorn of the Britannian students. Cornelia continues her hunt for Zero, destroying several rebel strongholds but finding no clues to his whereabouts. A stray cat slips into the Lamperouge residence on campus and takes off with Lelouch's Zero mask and a massive, comedic chase involving the entire student body follow. The Emperor of Britannia makes a speech during Prince Clovis' funeral on the virtues of inequality and the social battle for evolution and betterment.

Cornelia sets a trap for Zero at the Saitama Ghetto as she begins to clear another resistance cell. Despite C.C. trying to stop him, Lelouch takes the bait and goes to the aid of the Saitama resistance. His plan falls apart from a combination of Cornelia's superior tactics and the resistance cell's dissimilar motivations. Lelouch is cornered and nearly discovered, but C.C. dons his Zero costume and distracts the Britannian Army, allowing Lelouch to escape. Lelouch blames his defeat on the undisciplined rebels and resolves to build something better than a mere resistance unit.

A group from the Japan Liberation Front takes a hotel and its occupants hostage, including most of the Ashford Student council and a disguised Princess Euphemia. As such, Princess Cornelia cannot make a move against the hostage takers, which Lelouch realizes and takes advantage of. He mounts a rescue with Ohgi's resistance, using his knowledge of Euphemia to barter his way past Cornelia's blockade. As Lelouch speaks with the hostage takers, Suzaku sinks the floating hotel using the Lancelot. Using explosives set up by the resistance fighters, Lelouch derails any rescue efforts by the army so he can take credit for their rescue. Broadcasting publicly to the nation, he announces the creation and purpose of his Order of the Black Knights, "allies of justice" who will judge the world and protect the weak from "those with power".