After Aang receives a vision from his predecessor, Roku, and Zuko receives a letter from Iroh, they each learn about the relationship between Avatar Roku and Fire Lord Sozin; their childhood friendship, falling out, and Sozin's eventual betrayal of Roku to his death. Zuko discovers that Roku is his maternal great-grandfather. Iroh explains to Zuko that the legacy of the struggle between Roku and the latter's paternal great-grandfather, Sozin, lives on as the struggle between good and evil within Zuko himself.
Aang is visited in a dream by Roku, who tells him it is time the current Avatar learns about his past life's history with Fire Lord Sozin. Roku instructs Aang to travel to the former's old home, a deserted volcanic island, during the summer solstice. Upon arriving, Toph senses there is an entire village buried beneath the volcanic ash of the island's shore. As the solstice begins, Aang meditates and meets Roku in the Spirit World.
At the Fire Nation palace, Zuko is awoken by the noise of someone outside his bedroom door. He rushes out to see who it is and finds a scroll sitting on the floor, telling him he must find out about his great-grandfather's death in order to understand his own destiny.
Zuko visits a portrait gallery of the past Fire Lords the next day and is greeted by his sister Azula. He asks her if she knows how their great-grandfather Fire Lord Sozin died, and she tells him he died peacefully in his sleep. Zuko mulls over the mysterious message to himself, unsure of what it means. In frustration, he throws the parchment over a lamp and discovers a hidden message in the note, telling him to go to the Dragonbone Catacombs to find the secret history. After eluding Fire Sages, Zuko infiltrates the catacombs and finds "The Final Testament of Fire Lord Sozin", an autobiographical record of his life.
As Zuko reads the record, Roku shows Aang a flashback of his youth, showing him practicing his firebending with Crown Prince Sozin. Roku and Sozin were best friends. They even shared the same birthday, which they celebrated together. On their sixteenth birthday, Roku's identity as the Avatar was revealed by the Fire Sages. The two friends were forced to separate as Roku had to leave the Fire Nation to master the other three elements. Before leaving, Sozin gave Roku a gift: the hair ornament customarily worn by the Crown Prince of the Fire Nation. Roku wore it ever since.
Roku began his journey at the Southern Air Temple, training to master airbending. There he became friends with young Gyatso, a fact that surprises and delights young Aang. Roku tells Aang that some friendships are so strong, they can transcend lifetimes.
After several years mastering airbending, Roku traveled to the Northern Water Tribe where he learned waterbending. Though it was challenging to learn his natural opposite, after several years Roku mastered it as well. He journeyed to the Earth Kingdom, where he mastered the last remaining element with an earthbender named Sud before becoming a fully realized Avatar. After the twelve years of training, Roku had returned to the Fire Nation. His old friend Sozin had become the Fire Lord and greeted him warmly. Roku married his childhood sweetheart, Ta Min, with Sozin as his best man. During the wedding, Sozin took Roku aside and remarked that the Fire Nation was going through a period of great prosperity. With him as the Fire Lord and Roku as the Avatar, he believed the two of them could spread the Fire Nation's influence and create a prospering empire. Roku was greatly disturbed by this, telling Sozin that the four nations were meant to be kept separate and to abandon such thoughts.
Many years later, Roku discovered that Sozin had ignored his request and invaded the Earth Kingdom. He confronted Sozin and demanded he cease the expansion. Enraged, Sozin attacked Roku with a powerful display of firebending. Roku quickly disabled him and used the Avatar State to destroy the palace. The Avatar starkly warned Sozin he was sparing his life in recognition of their past friendship, but warned if he overstepped his boundaries again he would not hesitate to eliminate him.
Twenty-five years later, Roku's island was consumed in a volcanic eruption. Roku helped his island's villagers escape safely and tried to contain the eruption, but without success. So massive was the eruption that Sozin could see and feel it from a hundred miles away. Sozin flew to the island on his blue dragon to assist his old friend, and the two worked together to try and quell the volcano.
Their effort appeared successful, yet the poisonous gas from the volcano overwhelmed Roku and caused him to collapse to the ground. He called to Sozin for help, but Sozin realized the elimination of the Avatar would make all his future plans for the Fire Nation suddenly possible. He flew away on his dragon, leaving Roku and his dragon, Fang, to die from the torrent of volcanic ash. Shortly afterward, as Roku exhaled his last breath, Avatar Aang was born to the Air Nomads.
Zuko finishes reading Sozin's testament, which explains Sozin attacked the Air Nomads to eliminate the next Avatar, but the Avatar managed to escape somehow. Sozin spent the rest of his life searching fruitlessly to find the Fire Nation's greatest threat: "The Last Airbender." Zuko is disappointed by the ending, still not understanding how it concerns him.
Zuko goes to the Fire Nation jail tower and bursts angrily into Iroh's cell. He accuses Iroh of having sent him the message and demands the point, as the testament did not even reveal anything significant about Sozin's death. Iroh explains that the message did not refer to his father's grandfather, Sozin, but to his mother's grandfather, Avatar Roku. Zuko is alarmed by this, and Iroh further explains that their combined heritage is the reason Zuko is still conflicted over his destiny; whether he should destroy or assist the Avatar. Iroh gives Zuko the lost Crown Prince's ornamental hairpiece, telling him he alone has the power to redeem their family and the sins of Fire Nation by restoring balance to the world.
Awakening from his vision with Roku, Aang tells his friends the story. He believes Roku was trying to tell him that any person is capable of great good or great evil, and that every person should be treated as though they are worth giving a chance for redemption. Toph wonders if friendships can really transcend lifetimes; Aang and Katara hold her hand to assure her, yet Sokka is skeptical, saying there's no real scientific proof. Annoyed, Katara demands he just hold hands with the rest of them, which he quickly does.
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- Main article: Transcript:The Avatar and the Fire Lord
- Main article: Writing in the World of Avatar
- Main article: Avatar Extras for Book Three: Fire
- When Katara asks if the Spirit World has bathrooms, Sokka says it does not, which is something he found out in "Winter Solstice, Part 1: The Spirit World".
- The mistakes Avatar Roku alluded to when he appeared to Aang in "The Awakening" are elaborated upon in this episode.
- In the flashback where Roku waterbends his waterbending teacher through the Northern Water Tribe city, the large ice wall that surrounds the city had not yet been built.
- As Zuko reads the final parts of Sozin's last will, a Fire Nation cruiser is shown passing over the iceberg Aang was encased in, which would be cracked open in "The Boy in the Iceberg".
- It is revealed that Roku was Zuko's maternal great-grandfather.
- By extension, Zuko realizes that he is spiritually related to Aang.
- When the scene cuts to Aang and the gang on the cliff at Roku's island, Aang is seen standing up, but when the view zooms in, Aang is sitting down, cross-legged.
- When the Fire Sages bow down to the young Roku, Aang and Avatar Roku are not in the crowd. However, when everyone is bowing, Aang and Avatar Roku appear on the right.
- When Roku returns to the Fire Nation after completing his Avatar training, he enters the Royal Palace's Throne Room and sees Sozin sitting on the throne. In one brief shot, when Sozin is sitting down, part of the detail on the lower half of his robe is missing.
- During the scene depicting Roku's wedding, his hair and beard appear to be brown; they are black in the rest of the episode.
- During Roku and Ta Min's wedding, Sozin pulls Roku aside to speak with him. In one shot, the two of them can be seen on a balcony overlooking a large area resembling the area where the Royal Plaza is now located. If the area in this shot is in fact of the Royal Plaza's location before it was built, it should not be visible from the balcony, as the walls of caldera would be surrounding the Capital City, thus obscuring the location from view. The same can be said for the scene where an elderly Sozin watches the volcano on Roku's island erupt in the distance from the Fire Nation Capital.
- In one scene during Sozin and Roku's conversation, Sozin's collar turns red when it should be black.
- Entering the throne room to confront Sozin, Roku leaves the doors open, though when Sozin attacks Roku with a fire blast, they are closed.
- When Roku enters the palace to confront Sozin, his beard is graying. After Sozin attacks and he rises from behind Sozin, the beard is solid black. It reverts to graying when Roku enters the Avatar State.
- When Sozin first sees Roku's home island being burned, his face is shown with darker colored spots on it. However, the next close-up scenes of Sozin, particularly on the island, show his face without any spots or scars.
- When Avatar Roku goes into the Avatar State and begins to blow a hole in the other side of the volcano, his teeth are the same color as his skin.
- Sozin's dragon is blue. However, when the Fire Lord arrives at Roku's island to help him battle the volcano, the dragon is colored red.
- When Sozin talked about wiping the air temples, three burning views of the Southern Air Temple are shown instead of the different air temples.
- After Aang has told the gang about Roku and Sozin, Sokka's sword is missing, but the strap on his chest is still there. In the next frame his sword is shown on his back, but both the strap and the sword disappear for the rest of the episode when Aang talks about friendships.
- At the end of the episode, when Aang, Toph, Katara, and Sokka are holding hands, their backs are to the sea. In the final frame, they are facing the sea and in the opposite order.
- While in the Spirit World, Aang was bald and wearing his original airbender clothes again.
- If the visions of Roku's training in this episode are an example of usual Avatar training customs, it seems that while training in each of the other elements, Avatars will wear the standard clothing of that nation but return to their own when they have mastered all the elements. This is later confirmed in The Legend of Korra, as Avatar Korra is seen training airbending in traditional Air Nomad clothes, and at the beginning of the series is seen training firebending in Fire Nation armor.
- During Roku's wedding, guests in blue and green clothing can be seen, representing the openness of the four nations before the War.
- The Fire Nation colony that Roku and Fang see has a large wall, similar to that of Ba Sing Se.
- The stance and motion that Fire Lord Sozin uses to sink and redirect the volcano's heat energy directly mirrors the form that Iroh and Zuko use to channel and redirect the energy from lightning.
- When the Eastern Air Temple is seen burning, the sky seems to be orange due to the comet.
- This is the first chapter in Book Three that Iroh speaks. Iroh's long period without dialogue is believed by some to be related to the death of Iroh's Book One and Book Two voice actor, Mako, but on the DVD commentary, the creators had previously planned to have Iroh silent for the first few episodes.
- Roku said, during his wedding flashback, that as the Avatar, you get noticed by the ladies. Aang got this same advice from the other prisoners during his time in the Chin Village prison.
- This is the second time that Roku gave Aang a specific time and place to meet him, namely during the summer solstice at his homeland. This mirrors the episode "Winter Solstice, Part 2: Avatar Roku", where Aang had the chance to talk to him during the winter solstice at the Fire Temple on Crescent Island.
- Fire Lord Sozin justifies the invasion of the other nations as sharing wealth, a concept derived from Japanese Imperialism. In World War II, the Japanese Empire used an ideological construct, "Dai Toa Kyoeikan" ("The Great East Asia Co-Prosperity Sphere") to justify invading China and conquering the South Pacific, just as the Fire Nation has invaded the Earth Kingdom and Water Tribes.
- The relationship between Roku and Sozin, going from friend to enemy, is opposite to Aang's and Zuko's relationship, going from enemy to friend. Zuko and Aang were friends mainly after Zuko became Fire Lord and Sozin and Roku before.
- Roku's wife, Ta Min, makes her first and sole appearance in this episode.
- This episode is alternatively titled "The Avatar and the Firelord" on iTunes and the DVD.
- This is the third and final episode which title starts with "The Avatar". "The Avatar Returns" and "The Avatar State" are the earlier two, from Books One and Two respectively.
- The creators stated this episode to be one of the most complex of the series, with by far the most background designs of any episode.
- This is the last episode in the series in which Aang enters the Spirit World.