"Born in you, along with all this strife, is the power to restore balance to the world."
The sixth episode of the third season of the show, 'The Avatar and the Fire Lord' explores the history of the Avatar-verse and the unique relationship between Avatar Roku and Fire Lord Sozin. Similar to the episodes 'The Storm' and 'Zuko Alone', the episode is heavy in flashbacks, using them almost exclusively. Unlike the previous two episodes, however, these are not the memories of any of the main characters, and are all delivered post-mortem.
Aang receives a vision from Roku, telling him that he needs to learn of the events that led to the war, while Zuko gets a message that states he needs to learn about his great-grandfather's death to learn more about himself. The Gaang goes to Roku's old home, an island village that has been buried under volcanic ash. There, Aang enters a trance and meets Roku. Zuko, meanwhile, finds Fire Lord Sozin's last testament.
Originally, Roku and Sozin were once friends, who 'shared many things', including birthdays. On their 16th, the Fire Sages appear at the party and declare Roku the new Avatar. Roku is a bit unnerved about the new responsibilities he has, but before he begins his journey to learn the other three elements, Sozin gives him his headpiece as a gift of friendship.
Roku's first stop in his training is to learn airbending. At one of the Air Temples, he becomes friends with another trainee - a boy named Gyatso. Aang is amazed to learn this, and Roku assures him that "Some friendships are so strong that they can transcend lifetimes".
Roku travels all over the world mastering fire, air, water, and earth, and by the time he becomes a fully-realized Avatar, twelve years have passed. He returns home to the Fire Nation, where Sozin has become the new Fire Lord. But time has not caused their friendship to diminish one bit, and the two were glad to reunite. At Roku's wedding, Sozin mentions what a strange coincidence that the Avatar and Fire Lord are such close friends; they could do anything. He goes on to say that the Fire Nation is also in an era of peace and wealth, and that working together, they could share that wealth with the world... by expanding their nation.
Roku is against it, citing that the four nations should remain as just that, four. However, years later Roku learns that Sozin is going ahead with his expansion plans, and has occupied Earth Kingdom territory. Confronting the Fire Lord, the Avatar berates him for his actions, but Sozin attacks Roku. The battle ends quickly in Roku's favor, but he spares Sozin's life, warning that a second conflict will be Sozin's end.
Roku and Sozin do not see each other for twenty-five years. One night, however, a volcano erupts on Roku's island. As the rest of the villagers evacuate, Roku does what he can to fight against the tides of lava. However, the difficult task of fighting a volcano is made harder by the poisonous, volcanic gases. However, Sozin, having felt the rumbles of the eruption even from the hundred mile distance of his palace, comes to his aid. The two fight the eruption together, but Roku begins to succumb to the gases. Seeing his old friend in that state, Sozin comes to a decision;
Sozin: Without you, all of my plans are suddenly possible.
Sozin leaves Roku to his death, and somewhere in an Air Temple, a baby called Aang is born. Roku tells Aang to "Make sense of our past, Aang, and you will bring peace to the world." Aang's trance ends.
Zuko reads that Sozin began making preparations for war - the combination of the coming comet and Roku's recent death being the perfect opportunity. Knowing that the Avatar will be reborn as an Air Nomad, Sozin wiped out the Air Temples. However, he never found the Avatar, and spent the rest of his life looking for the Fire Nation's greatest threat... the last airbender.
Zuko goes to Iroh's cell and demands an explanation, that the testament didn't explain any new information on how Sozin died. Iroh explains that the note he sent was not referring to his father's grandfather, but his mother's... Avatar Roku. Iroh compares on how Zuko bearing the blood of both the past Avatar and the then-Fire Lord parallels the conflict of good and evil inside him, and that Zuko's legacy can resolve the conflict and restore the Fire Nation's honor. He then gives Zuko a gift, something that's supposed to be worn by the crown prince - the headpiece Sozin gave Roku.
Meanwhile, the Gaang try to figure out the point of Roku's story. Aang concludes that it was to show that everyone has the potential for good or evil in them - Roku himself was Fire Nation, and he tried to stop the war. The Fire Nation needs to be treated like they can change for the better. And the story might also be about friendships...