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".

Told mostly from Kallen's perspective, her rocky relationship with her biological mother is revealed. The Black Knights are sent on a mission to take out the drug "Refrain", and find themselves to be popular among the Elevens. Kallen's mother is found among the Refrain users, and is arrested following the Black Knights' destruction of the drug stockpiles. Kallen promises to have changed the world by the time she is released.

Cornelia and the Britannian Army heads to Narita, hoping to destroy the remnants of the Japan Liberation Front. Lelouch has his Knights follow her in the hope of capturing her alive so he can question her about his mother's death. As the Britannian forces close in on the Front's headquarters, Kallen utilizes her new Knightmare Frame, the Guren Mk-II, to cause a massive landslide that takes out much of the Britannian forces.

The Black Knights' assault on the Britannian Army in Narita proves successful, and Cornelia is cornered by Kallen and Lelouch. The army is pinned down by the Japan Liberation Front when they attempt to assist her. At Euphemia's request, Suzaku rushes onto the battlefield in the Lancelot to rescue Cornelia. He saves Cornelia and manages to corner Lelouch. C.C. comes to Lelouch's rescue by mentally traumatizing Suzaku, but is injured when Suzaku begins firing wildly. As he takes care of her, Lelouch inadvertently learns her real name. The Black Knights retreat from the conflict in victory, leaving the Japan Liberation Front and the Britannian Army to fight amongst themselves.

Zero and the core members of the Black Knights meet with Taizō Kirihara, one of the most powerful people in the Japanese underground and an acquaintance of Lelouch from his younger days at the Kururugi Shrine. Lelouch deftly displays his tactical prowess and wit before Taizō and reveals his true identity to the man to gain his trust and support. When Lelouch returns to Tokyo, Shirley informs him of her father's death, indirectly caused by Zero during the Battle of Narita.

Kallen and Lelouch separately reflect upon the deaths of innocents in the Narita campaign. Shirley secretly follows Lelouch and witnesses the battle between the Japan Liberation Front and the Britannian Army, which Lelouch ends by detonating the Japan Liberation Front freighter as the Britannian Knightmares begin to board it, killing all personnel on board and giving the Black Knights the element of surprise. Suzaku attacks Lelouch, easily overpowering his weaker Burai with the Lancelot, forcing him to eject. He crash-lands near Shirley and is knocked unconscious. Shirley prepares to kill him to avenge her father, but hesitates when his mask falls off, revealing to her that Zero is Lelouch.

Lelouch believes Shirley may have discovered his identity as Zero, so he and C.C. pursue her to Narita. Ohgi explores the battlefield of the previous night, discovering the wounded Villetta Nu left for dead out of sight. Lelouch encounters Mao, a young man who also has the power of Geass. Mao uses his mind-reading power to play on Shirley's insecurities, manipulating her into shooting Lelouch, but she falters at the last moment and C.C. sends Mao away temporarily. Since Shirley is being torn apart by grief and guilt, Lelouch uses his Geass to erase her memories of him.

C.C. explains her history with Mao, and how his inability to turn his Geass off drove him insane. An injured Villetta wakes up in Ohgi's apartment and he discovers she has selective amnesia. C.C. leaves Lelouch to take care of Mao herself, but Mao gains the upper hand when he incapacitates her. Lelouch utilizes his intellect to fool Mao and rescue C.C. from him, using the Britannian Police to surround and shoot Mao. Safe on a rooftop away from the scene, Lelouch affirms his contract with her, promising to not fall victim to his power as Mao did.

C.C. departs as a messenger to the Chinese Federation and Milly has a marriage interview with Lloyd Asplund, creator of the Lancelot. Mao returns to kidnap Lelouch's sister, Nunnally, and holds her hostage in the Ashford Academy sewers. Lelouch and Suzaku work together to rescue her, with Suzaku's physical adeptness and the clever use of Lelouch's Geass proving to be beyond Mao's planning. Before he can be arrested, Mao reveals that Suzaku killed his own father, and in return Lelouch silences him with his power. C.C. returns to the campus and finally ends Mao's life as Suzaku crumbles to the ground, confronted with his actions after years of secrecy.