Code Geass doesn’t have official arcs listed but we can divide the series based on the story structure into the following list.
1. Introduction (New Knight) Arc
Episodes: 1-4
In 2010, the Holy Britannian Empire declares war on Japan and conquers it. Seven years later, a teenager named Lelouch Lamperouge becomes involved in a conflict between anti-government terrorists and the Britannian Army in the Shinjuku Ghetto. During the fight, a mysterious green-haired girl gives Lelouch the power to make others do as he commands, which he uses to hijack an enemy Sutherland and lead the rebels in a counterattack against the Britannian Army.
The battle ends when Viceroy Clovis la Britannia agrees to a cease-fire under Lelouch’s coercion. The Special Envoys also release a superior Knightmare Frame called the Lancelot, which is piloted by Suzaku Kururugi.
2. Great Game of Shogi Arc
Episodes: 5-13
Lelouch is surprised to find out that C.C. is alive and living with him. Suzaku, who has been cleared of all charges, meets Princess Euphemia and becomes involved in a conflict involving purebloods. Princess Cornelia is trying to capture Zero, who is now the Holy Britannian Empire’s most wanted man.
Suzaku faces hostility from other Britannian students when he transfers to Ashford Academy, and Cornelia continues to search for Zero without success. A stray cat takes Lelouch’s Zero mask, leading to a humorous chase involving the student body. The Emperor gives a speech about inequality at Prince Clovis’ funeral.
3. Mao’s Life Arc
Episodes: 14-16
Lelouch and C.C. pursue Shirley after Lelouch becomes suspicious that she has discovered his identity as Zero. Along the way, they encounter Mao, who has the power of Geass, but Lelouch is able to use his own Geass to outsmart him and rescue C.C.
After Mao kidnaps Nunnally and holds her hostage, Lelouch and Suzaku work together to rescue her, during which Mao reveals that Suzaku killed his own father. Lelouch silences Mao with his Geass and C.C. kills him, while Suzaku is left to deal with the revelation of his actions.
4. Operation Downfall Arc
Episodes: 16-21
Euphemia judges artworks at the inauguration ceremony of Clovis’ memorial art museum. Lelouch helps Tohdoh’s comrades rescue their leader and convinces Tohdoh to join the Black Knights. During the mission, they encounter Suzaku in the Lancelot and Lelouch misses the opportunity to kill him.
Euphemia announces Suzaku will be her knight, and Lelouch establishes the hierarchy of the Black Knights. He plans to capture Suzaku and the Lancelot at a military base on Shikine Island, but the mission is interrupted by the arrival of Prince Schneizel, who orders his floating battleship, the Avalon, to bombard the island and kill Zero.
5. Uprising Arc
Episodes: 22-25
During the commemoration ceremony for the Specially Administrated Zone, Lelouch and Euphemia discuss Japan’s future, and Lelouch halts his rebellion. However, he loses control of his Geass and Euphemia orders a massacre of the attendees.
Lelouch declares Euphemia’s Specially Administrated Zone a trap to lure out the Black Knights and commands his troops to defeat the Britannian forces and protect the Japanese. He shoots and mortally wounds Euphemia in front of Suzaku, and announces his plan for the creation of the United States of Japan.
6. Back to Basics Arc
Episodes: 26-32
One year after the First Black Rebellion, Lelouch has lost his memories and is living a normal school life. However, the Order of the Black Knights attacks Babel Tower to retrieve him, and C.C. restores his memories, leading Lelouch to resume his identity as Zero. Suzaku declares he will personally kill Zero, and Lelouch directs the Black Knights in a counterattack against the Britannian forces.
Zero broadcasts his plan to recreate the United States of Japan and the Britannian military retreats, but Rolo, the pilot of the Vincent, possesses the power of Geass and confronts Li Xingke at the consulate.
7. Journey to The West Arc
Episodes: 33-36
The Black Knights question Zero’s decision to cooperate with the formation of the Special Administrated Zone of Japan, and the Britannian Army remains wary of his intentions. Zero requests to be exiled from Area 11 in exchange for delivering one million Elevens to the SAZ, and during the opening ceremony, every participating Eleven dresses as Zero, leading Suzaku to exile all one million “Zeros.”
Lelouch learns that the Chinese Empress Tianzi is being forced into a political marriage with First Prince Odysseus, which is being orchestrated by the High Eunuchs in exchange for land from Second Prince Schneizel. Li Xingke initiates a coup to overthrow the Eunuchs, but Zero appears and takes the Empress hostage. Meanwhile, V.V. readies Jeremiah Gottwald against Lelouch and C.C.
8. Honour of Jeremiah Gottwald Arc
Episodes: 37-39
Lelouch tries to undo the damage caused by Sayoko’s portrayal of him as a playboy, and Shirley discusses recent events with him. Knight of One Bismark Waldstein informs the Emperor of a meeting between the princes and princesses about the Chinese Federation, and Cornelia confronts General Bartley about Geass. Suzaku and Lloyd discuss the Guren, and Nunnally visits Kallen in prison.
Jeremiah arrives in Area 11 and uses his Geass Canceller, restoring Shirley’s memories of Lelouch’s identity as Zero. Lelouch heads to the train station to check on the G Trains, and Jeremiah searches for him at Ashford Academy. Lelouch reveals his motivation for avenging his mother to Jeremiah, and Shirley reconciles her restored memories and asks Suzaku to forgive Lelouch. Rolo wounds Shirley when she reveals knowledge of Nunnally, and Lelouch stays with her in her final moments.
9. F.L.E.I.J.A Arc
Episodes: 40-44
Suzaku decides not to use Refrain on Kallen, and Ohgi and Villetta confess their love for each other. Nina and the In Vogue staff successfully test a bomb, and Lelouch confronts the Emperor at the Sword of Akasha. He learns that his father has taken V.V.’s “Code of Immortality” and become immortal, and C.C. reveals her desire to die.
Lelouch witnesses her past in “C’s World” and returns to reality, attacking the Sword of Akasha and rescuing C.C., who has lost all her memories except those before receiving the power of Geass. The United Federation of Nations declares war against Britannia to liberate Japan, and the Knights of the Round are transferred to Japan. Nina asks to have the Lancelot equipped with a F.L.E.I.J.A. warhead, and the Emperor challenges Zero to a battle for the world. Lelouch admits to Suzaku that he is Zero and asks him to rescue Nunnally, agreeing to meet at the Kururugi Shrine.
10. Ragnarok Arc
Episodes: 45-46
Lelouch and Suzaku head to Kamine Island separately to face the Emperor, and Suzaku strikes a deal with Schneizel to become the Knight of One in exchange for assassinating the Emperor. Lelouch takes over much of the Britannian military forces on the island, and the leaders of the Black Knights declare Zero dead. Anya is taken over by Marianne and restores C.C.’s memories, and Suzaku is held at bay by Bismarck while Lelouch marches into the Sword of Akasha unopposed.
Inside, Lelouch learns the truth about his mother’s death from the Emperor and Marianne, and Suzaku and C.C. reveal their intention to initiate Ragnarök. Lelouch defeats his father with the help of the collective unconscious mind of humanity and dissolves his parents from existence.
11. Demon Emperor Arc
Episodes: 47-50
Lelouch becomes the leader of Britannia and abolishes the aristocracy and Area system. He plans to join the U.F.N., but the group demands limits on his voting power. In response, Lelouch has Suzaku take U.F.N. leaders hostage and has the military surround Japan. Lelouch’s brother, Schneizel, reveals that their sister, Nunnally, is alive and declares herself an enemy of Lelouch and Suzaku.
Schneizel unveils his plan to use the Damocles, a flying fortress, to threaten the world with annihilation in order to enforce peace. Lelouch and Suzaku fight against Schneizel, who is backed by the Black Knights and the remaining Knights of the Round. Lelouch tries to destroy the Damocles, but it remains unassailable and carries F.L.E.I.J.A. warheads.