Long ago, the four nations lived in harmony. Then everything changed when the Fire Nation attacked. Only the Avatar could wield all four elements, but when the world needed him most, he vanished. A hundred years passed, and the Fire Nation continued its ruthless conquest.
Katara and Sokka were fishing to feed their tribe. Katara practiced waterbending, but she still struggled to control it. She accidentally soaked Sokka, and they argued the way they always did. During the fight, she broke a giant iceberg, revealing a boy trapped inside. When she freed him, a bright beam of light shot into the sky. Nearby, Prince Zuko saw the light and told his uncle it must be the Avatar, ordering the ship to change course.
The boy introduced himself as Aang, and he was much friendlier than expected. He checked on his frozen flying bison, Appa. Katara realized Aang was an airbender, and he flew them home on Appa. A flashback revealed that Aang had frozen himself to survive a storm. Katara introduced Aang to her tribe, where only children and the elderly remained, because the men, including her father, were away fighting the Fire Nation.
Her grandmother said they had not seen an airbender in a hundred years and warned Katara not to get too excited. Meanwhile, Iroh warned Zuko that he was too impatient and too obsessed with capturing the Avatar. Back at the village, Katara asked Aang to teach her waterbending, but he only knew airbending, so he offered to take her to the North Pole to learn from the Northern Water Tribe.
Later, they discovered an abandoned Fire Navy ship. Despite Katara's warning, Aang convinced her to explore it. He revealed that he had never seen war, and Katara explained that he had been frozen for over a hundred years, meaning everyone he knew was gone. They accidentally triggered a flare trap, which Zuko spotted, and he realized he had finally found the Avatar. Sokka blamed Aang for putting the village in danger and banished him.
Katara wanted to leave with Aang for the North Pole, but the tribe refused. Just then, Zuko attacked the village. Sokka was quickly overwhelmed until Aang returned, saved everyone, and revealed himself as the Avatar. When Zuko threatened the villagers, Aang surrendered to protect them. Sokka and Katara immediately set out to rescue him. Aang escaped from Zuko's ship with airbending, and the others arrived just as he fought Zuko.
After falling into the water, Aang entered the Avatar State and blasted everyone away. The kids escaped by freezing Zuko's ship in ice. Humbled by his defeat, Zuko vowed never to underestimate the Avatar again. Later, Aang admitted that he had never wanted to be the Avatar, and with Katara's encouragement, they decided to travel to the North Pole so he could master waterbending and stop the war.
Excited to return home, Aang learned that a hundred years had passed. Meanwhile, Zuko and Iroh stopped at a port to repair their ship, where Commander Zhao overheard them and invited them for tea to learn more. The kids arrived at the Southern Air Temple, but it was completely empty. Inside the sanctuary, Aang discovered statues of the past Avatars and learned that each Avatar is reborn into the next nation.
While chasing a lemur, he found Monk Gyatso's skeleton surrounded by fallen Fire Nation soldiers. Heartbroken, Aang entered the Avatar State, causing Avatar statues around the world to glow and announce his return. At the same time, Zhao learned that Zuko had found the Avatar but let him escape. Claiming the Avatar for himself, he challenged Zuko to an Agni Kai. Zuko won the duel, proving he was the stronger firebender.
Katara comforted Aang and promised that they would be his new family. The group named the lemur Momo and continued their journey. The trio landed on an island where Aang rode a koi fish to impress Katara. A sea monster suddenly attacked, and female warriors captured them. The villagers explained that they were on Kyoshi Island, named after the Avatar from four hundred years ago.
Aang proved he was the Avatar by showing his powers. News of the Avatar spread across the island and reached Zuko. Aang enjoyed all the attention while Katara warned him not to let fame go to his head. She was also a little jealous of all the girls around him. Sokka challenged the Kyoshi Warriors, was defeated, and asked Suki to train him.
Zuko attacked the island. The warriors defended their home, and Aang decided to leave because the village was suffering because of him. Before departing, Sokka apologized to Suki, who kissed him goodbye, and the trio escaped on Appa. The kids arrived in Omashu, the Earth Kingdom's second largest city. Aang disguised himself as an old man and remembered playing with his old friend Bumi by riding the city's delivery chutes.
When they tried it again, they were arrested and brought before the king. The king recognized Aang and captured the group, taking Sokka and Katara hostage. He then gave Aang three dangerous challenges: finding a key, finding his pet, and fighting him. Aang completed all three challenges. Although the king acted crazy, he was an incredibly powerful earthbender. In the end, Aang discovered that the king was Bumi, who freed his friends and reminded him to master all four elements to defeat the Fire Lord and save the world.
While searching for food, the group met Haru, a young earthbender living in a Fire Nation occupied village where all earthbenders were sent to prison. His father, Tyro, had been captured after standing up to the Fire Nation. Katara related to him. Later, Haru saved an old man trapped in a mine with earthbending, but the man betrayed him, and Haru was arrested.
With Aang and Sokka's help, Katara got herself captured and taken to the prison ship, where she reunited with Haru and Tyro. The prisoners had lost hope and refused to fight, but Katara inspired them while Aang brought coal so they could earthbend again. Together, they defeated the guards and escaped. Before leaving, Haru and Tyro thanked Katara, but she realized her necklace was missing.
Zuko had it. The group reached a forest burned down by the Fire Nation. While Aang felt helpless, Katara reminded him that it would grow back. A villager begged the Avatar for help, because every sunset a spirit named Hei Bai attacked his village and kidnapped its people. As the bridge between worlds, Aang decided to help. Meanwhile, Iroh was captured by Earth Kingdom soldiers while relaxing in a hot spring.
Hei Bai attacked again, kidnapped Sokka, and knocked Aang unconscious. While Iroh secretly left clues for Zuko, Aang awakened in the spirit world, where a dragon spirit guided him to a temple. There he saw a vision of a powerful comet and learned that he could speak with Avatar Roku during tomorrow's winter solstice. Aang returned to the real world. While searching for Iroh, Zuko spotted Aang but chose to save his uncle instead.
Zuko reunited with Iroh and defeated the Earth Kingdom soldiers. Aang found Hei Bai again and realized he was the forest spirit, enraged because his home had been destroyed. After receiving an acorn, Hei Bai calmed down and freed the captured villagers, including Sokka. Realizing time was running out, Aang decided that they must travel to a Fire Nation island to meet Roku. They tried to enter the Fire Nation but were attacked by Zuko, and by Zhao too.
They reached the Fire Sage temple, where the sages attacked them. One of them, Shyu, secretly helped them reach Roku's sealed sanctuary by tricking the others into opening it. Zuko arrived and captured Aang, but he escaped into the sanctuary while Zhao captured everyone else. Aang met Avatar Roku, who revealed that a comet had given the Fire Lord the power to begin the war a hundred years ago.
It would return by the end of summer, and Fire Lord Ozai planned to use it to win the war once and for all. Aang had to master all four elements before then. Afterward, Roku took control of Aang's body, freed everyone with powerful firebending, and destroyed the temple so they could escape, while Zhao arrested the fire sages. Later, Katara tried teaching Aang waterbending, but he learned it effortlessly, leaving her frustrated.
In a nearby town, Aang bought a bison whistle while Katara secretly stole a waterbending scroll from some pirates. Aang quickly mastered the techniques and even tried teaching Katara, frustrating her. Meanwhile, Zuko teamed up with the pirates to capture Aang. That night, Katara snuck away to practice, but she was captured by Zuko, who offered her mother's necklace in exchange for the Avatar.
The pirates captured Aang and Sokka, but they betrayed Zuko after learning who Aang was, leading to a three-way fight. Momo freed Katara, and the group escaped using their new waterbending. The pirates chased them over a waterfall, but Aang saved everyone by summoning Appa with the whistle. Sokka revealed that he had recovered the scroll, and the journey continued.