Book One: Water

In the first season, Zuko is introduced as the main antagonist, the exiled son of Fire Lord Ozai who is attempting to capture Avatar Aang in hopes of reclaiming his honor. Throughout the season he pursues the Avatar, at times both helped and hindered by his uncle, Iroh. Zuko competes in his quest against Zhao, an ambitious Fire Nation admiral who is also attempting to capture the Avatar for political gain. When Zhao captures Aang, Zuko disguises himself as "The Blue Spirit" and rescues Aang from Zhao's fortress to prevent him from accomplishing his goal. Later, Zuko escapes an assassination attempt plotted by Zhao. During the invasion of the Northern Water Tribe, Zuko captures Aang, but both nearly die in a snowstorm before they are rescued by Aang's friends, Katara and Sokka. Zuko encounters Zhao and attacks him, but Aang, in the Avatar State, abducts Zhao before the fight is concluded. Zuko attempts to save Zhao from Aang, but Zhao is too proud to accept Zuko's help. Zuko survives the battle with help from Iroh, who betrayed Zhao to protect the ocean and moon spirits. In retaliation for Iroh's betrayal and Zuko's failure to capture the Avatar, Ozai assigns Zuko's younger sister Azula to capture Zuko and Iroh.

Book Two: Earth

In the second season, Zuko and Iroh travel the Earth Kingdom as impoverished refugees, pursued by Azula. After briefly quarreling and parting ways with Iroh, Zuko reunites with his uncle against Azula. After Azula wounds Iroh, Zuko nurses Iroh back to health and Iroh teaches Zuko the rare firebending technique of redirecting lightning. Zuko and Iroh travel to the walled city of Ba Sing Se and find employment at a tea shop in the slums. Zuko reluctantly adapts to his new life until he learns of Aang's presence in Ba Sing Se and attempts to steal Aang's pet bison Appa. However, Iroh convinces Zuko to abandon his quest and let Appa go. Azula infiltrates the city to stage a coup and arranges a trap for Zuko and Iroh. Imprisoned together with Aang's friend Katara, Zuko is rescued alongside her by Aang and Iroh, only to be convinced by Azula to betray Iroh, Katara and the Avatar and finally regain his honor. In the resulting fight, Aang is mortally wounded, though later revived by Katara, and Iroh is arrested for helping Aang, Katara escapes from the conquered city with Aang.

Book Three: Fire

In the third season, Zuko returns to his country a hero thanks to Azula crediting him with killing the Avatar. Zuko suspects that Aang survived and hires an assassin to ensure Aang's death to cover up Azula's lie. He resumes his romantic relationship with his childhood friend Mai, but continues to feel guilty and uncertain of his decisions. After he learns that his mother's grandfather was Avatar Roku, Aang's predecessor, as well as Azula and Ozai's plan to burn the Earth Kingdom to the ground, Zuko decides to turn his back on the Fire Nation and help Aang overthrow Ozai. He confronts his father about his cruel treatment and the Fire Nation's destructive effect on the world, before leaving to join Aang. Although they initially reject him, Aang and his companions grow to accept Zuko as a member of the group after he proves his loyalty to them. He trains Aang in firebending, and the two journey together to meet the last dragons, from whom they learn that firebending can be powered by the energy of life, rather than rage.

When Zuko is finally reunited with Iroh in the series finale, he tearfully apologizes for his mistakes and gets his uncle's forgiveness. Refusing the throne himself, Iroh suggests that Zuko become the next Fire Lord, citing Zuko's "unquestionable honor." Arriving at the Fire Nation capital with Katara, Zuko duels Azula for the throne. When Azula faces defeat, she forces Zuko to place himself in the path of a lightning bolt aimed at Katara, wounding him. Katara defeats Azula and heals Zuko. After Aang defeats Ozai and wins the war, Zuko becomes the new Fire Lord, promising world order and harmony with the aid of the Avatar.

The Promise

In The Promise, following Zuko's crown ceremony, seventeen-year-old Zuko asks Aang to promise to kill him if he ever becomes like his father. Aang reluctantly agrees. One year later, Zuko is working with the other nations to return captured colonial lands to the Earth Kingdom as part of the Harmony Restoration Movement. Kori Morishita attempts to assassinate Zuko in retaliation for forcibly relocating the multicultural residents of the Fire Nation colonies. Zuko travels to Kori's hometown, and Fire Nation Colony, Yu Dao, where he meets people of Fire Nation and the Earth Kingdom ancestry living and working together. Zuko becomes conflicted and consults with the imprisoned Ozai, who advises Zuko that whatever decision he makes is inherently the correct choice because he exercises the power of the Fire Lord. Zuko decides to break from the Harmony Restoration Movement. The Earth King responds with a military force of his own and Zuko is forced to lead troops to Yu Dao to protect the Fire Nation civilians. In the battle, Aang and Zuko clash. However, Katara convinces the Earth King that the colonies are neither Fire Nation nor the Earth Kingdom, but rather a multicultural people. Peace is restored, and Zuko realizes that forcing Aang to make the promise was a way of distancing himself from his responsibilities as Fire Lord. He releases Aang from the promise, and the two reestablish their friendship and trust.

The Search

In various flashbacks in The Search, it is revealed that Zuko's mother, Ursa, was previously engaged to a man named Ikem in her hometown of Hira'a, but had to leave him after being pressured into marrying Prince Ozai. Even after Zuko's birth, Ursa continued to write letters to Ikem, but Ozai intercepted the letters. Ursa, suspecting the interception, writes a letter claiming Zuko is Ikem's son rather than Ozai's. Ozai confronts her, and although he believes Zuko is truly his son, he retaliates by claiming to have ordered Ikem to be killed and stating he will treat Zuko as if he were actually Ikem's son.

A later flashback reveals that Ursa was responsible for creating an untraceable poison used by Ozai to kill Fire Lord Azulon in order to protect Zuko from Azulon. Ozai, fearing that Ursa would one day use the same poison against him, banishes Ursa from the palace the same night.

In the present, Azula reveals to Zuko she had access to the letters intercepted by Ozai. She uses her knowledge of their contents as leverage to be allowed to come with Zuko to find Ursa. Zuko and Azula travel with Aang, Katara, and Sokka to Hira'a. On the journey, Zuko tries to be more compassionate towards Azula, who suffers from erratic behavior and hallucinations following her mental breakdown. Zuko discovers the letter from Ursa stating that Zuko is Ikem's son, and begins to doubt if he is the rightful Fire Lord. Azula manipulates these doubts to weaken Zuko's resolve to be Fire Lord.

Zuko and Azula discover that Ursa is alive and married to Ikem, having had her memories erased and appearance changed by a spirit to relieve her of the pain of losing her children. Zuko is satisfied to know that Ursa is happy and is prepared to leave Ursa in peace, but with Ikem's encouragement, reveals to her that he is her forgotten son. Zuko also says that he is Ikem's son, but Ikem says that this is not possible, implying that he and Ursa never had intercourse before her marriage to Ozai. Azula attacks Ursa and Zuko and tries to persuade Zuko to give up the throne. Zuko reaffirms his commitment to his destiny as Fire Lord as well as their relationship as brother and sister. Azula's emotional and mental resolve breaks, and she flees into the forest.

Ursa chooses to have her memories and appearance restored by the spirit. Ursa reveals her past to Zuko, including that the letter claiming he was Ikem's son is false and that he is the rightful Fire Lord.

Smoke and Shadow

Weeks later, in Smoke and Shadow, Azula secretly manipulates Ozai loyalists into taking actions against Zuko, including an assassination attempt and kidnapping schemes that target Zuko's family and allies. In response, Zuko begins making harsher decisions in an attempt to preserve security and authority. Zuko's decisions are criticized by his allies and citizens as excessive. This leads to protesting and rioting within the Fire Nation capitol. Zuko and his allies eventually discover Azula's plot. Before escaping, Azula says she has accepted Fire Lord Zuko's claim as rightful Fire Lord, and has also accepted her own destiny to shape Zuko into becoming a stronger and more ruthless leader. In a speech, Fire Lord Zuko apologizes to his people and asks for their forgiveness, patience and trust as he tries to become a better leader.

Book One: Air

In The Legend of Korra, the multicultural region created in The Promise has developed into the United Republic of Nations. Lord Zuko, now eighty-seven years old, has abdicated the throne in favor of his daughter, Izumi, and travels the world on his dragon Druk as an ambassador for peace. He is commemorated by a statue at Republic City's Central City Station. His grandson Iroh (also voiced by Dante Basco) serves as a general of the United Republics' armed forces.

Book Three: Change

In Book Three: Change, it is revealed that Zuko worked with the Order of the White Lotus to keep major threats in check in the years before Avatar Korra was able. Most notably, he worked alongside Sokka, Aang's son Tenzin, and Korra's father Tonraq to thwart an attempt by the criminal Zaheer and his Red Lotus followers to kidnap Korra when she was a child. Fourteen years later, Zuko learns that Zaheer has become an airbender due to the harmonic convergence and escaped from prison. He enlists Tonraq and Korra's cousins Desna and Eska to keep Zaheer's partner P'Li from escaping, but fails to prevent P'Li from rejoining Zaheer. After Zaheer assassinates the Earth Queen, Zuko returns to the Fire Nation to protect Izumi. Zuko briefly counsels Korra before he departs and is surprised to learn that the Avatar has spoken to his long-deceased uncle in the Spirit World. After Korra defeats Zaheer, Zuko attends Jinora's airbending mastership ceremony and expresses concerns about the lingering threat of the Red Lotus.

Book Four: Balance

Zuko makes a final non-speaking appearance in Book Four: Balance, attending the coronation of Prince Wu along with Izumi.