Katara and Aang are packing up their campsite while Aang tells her about the astounding beauty of the Southern Air Temple; being the only home he has known, he is excited to return to it. However, Sokka remains in his sleeping bag; Aang tries to convince him to get up, but Sokka ignores his request. Ozai was still resting in Appa's saddle, his arms curled protectively around his baby bump. Too anxious to wait, Aang picks up a stick and slides it across Sokka's sleeping bag, telling Sokka in a faux-frantic tone, "There is a prickle snake in your sleeping bag! Watch out, Sokka!".

Terrified and screaming, Sokka leaps to his feet, still in the sleeping bag; only when he topples back to the ground face-first does he realize that he has been tricked. Katara and Azula suppress a laugh, while Zuko was laughing outright as Aang declares, "Now, we can leave since Sokka is now wide awake." This just caused Zuko to laugh even harder.

Off the coast of the southwestern Earth Kingdom, Lady Bellatrix docks at a Fore Nation controlled harbor, her ship needing substantial repairs after her recent encounter with Ozai. Zuko wants the repairs done quickly so they will not lose the Avatar's trail, and warns her brother not to mention the Avatar himself during their stay so that no other firebenders will realize what has happened.

Zhao walks up to Lu Jing and Bellatrix, welcoming them to his harbor, while informing them about his promotion from a captain to a commander. Seeing how damaged their ship is, Zhao inquires as to the cause, and Bella and Lu Jing invent a story about a collision with an Earth Kingdom ship. Unconvinced, Zhao invites the pair of them for a drink. Bellatrix attempts to reject the offer, but Lu Jing scolds her and, telling him that he owes Zhao a certain measure of respect, accepts the invite on behalf of his younger sister.

Nearing the Patola Mountain Range, a hungry Sokka finds out that Aang used all the blubbered seal jerky he had brought to start their campfire the night before, leading him to remark that it was "no wonder the flames smelled so good." Katara is trying to keep Aang from getting too riled about their visit to the air temple and she tells him to expect the possibility that his people have been killed by the Fire Nation. However, Aang refuses to believe the possibility. He believes that reaching an air temple is utterly impossible without the use of a flying bison and maintains that since the Fire Nation had no such assistance, they would not have been able to reach the temple. Ozai, on the other hand, was more worried about Aang's emotional reaction than the aftermath of the genocide. Approaching the top of the mountain, which pierces the clouds below, they look down on the Southern Air Temple.

Over tea in his tent, Commander Zhao outlines the Fire Nation's plan for conquering the Earth Kingdom to Bella and Lu Jing. When Bellatrix voices her bitter skepticism of the plan, calling her client a fool for believing the Earth Kingdom will capitulate quickly, Zhao inquires about the bounty hunter's search for the Avatar, to which Bella replies, "I have, thus far, been unsuccessful,". The commander remarks he did not expect anything different, but he persists in his pursuit of information from Bella, unsatisfied by her answer.

After a brief, heated exchange, the bounty hunter rises and attempts to leave, only to be detained by Zhao's guards at the entrance to the tent. One of Zhao's soldiers walks in and reports that he interrogated Bella's crew and found out that the latter had the Avatar, but let him escape. Smiling smugly, the commander asks the defeated bounty hunter to explain how her ship was damaged again.

Sokka, still hungry, is berated by Katara and Azula for being one of the first outsiders to see an airbender temple and thinking only about food. Aang starts to give Katara, Azula, Zuko, Ozai, and Sokka a tour of the temple, but failing to see any other airbenders causes him to quickly become disheartened. To take his mind off it, Sokka volunteers to play a game of airball with him.

As Sokka cannot bend air, Aang subsequently defeats him soundly in the game, the ball, propelled by airbending, hitting Sokka in the stomach and sending him flying backward into a pile of snow at the base of a very short abutment of earth. Lying on the ground, Sokka happens upon an archaic Fire Nation soldier's helmet; after pointing it out to a shocked Katara and horrified Zuko, Azula, and Ozai, they agree to show it to Aang.

However, as Aang begins to walk over, Katara suddenly changes her mind and waterbends a mass of snow downward, covering the helmet and Sokka with snow. She subsequently tells Aang, "The reason I called you over was to show you a new waterbending trick I learned,". Believing the misdirection, Aang comments positively on Katara's ability to bend snow before eagerly resuming the tour, wishing to show them the rest of the temple.

As Aang moves out of earshot, Ozai, after clearing the snow from Sokka's face, tells Katara, "Sweetheart, trying to shield Aang from the reality of the genocide committed against his people is a mistake. He has to find out eventually in order to grieve and heal properly,; to which Katara replies sadly, "Aang would be devastated if he knew firebenders invaded his home." Following Aang, they come to a tall statue of an airbender monk; Aang, bowing respectfully, introduces the man as Monk Gyatso.

In a flashback, Gyatso is teaching a young and disinterested Aang, the secrets of making fruit pies. When Aang tells a curious Gyatso, "I think the monks may have erroneously identified Aisling as the Avatar", Gyatso tells Aang, "The only mistake the monks made was telling her of her true identity before she reached the age of sixteen." He reassures Aang, saying, "You will be fine, Aang, but you must focus on the present. We may not be able to change the past, but our decisions in the present can effect the future. Once you are old enough to enter the air temple sanctuary, you will meet someone who will help you to help the next avatar to become a better Avatar."

A reassured and much happier Aang assists Gyatso in airbending the freshly baked fruit pies, sending them soaring through the air and directly onto the other elder monks' heads while they are meditating. The monks are promptly pounced on by winged lemurs, which slurp the pie off the monks' heads. The master and student laugh at their little prank, and Gyatso congratulates Aang on his perfected aim. He whispers into Aang's ear, "I love you, son."

Returning to the present, Aang presses onward toward the air temple sanctuary, recalling Gyatso's words about the mysterious entity within; he tells his friends that there is someone whom Ozai is ready to meet. Arriving at the grand, ornate doors to the sanctuary, Katara expresses doubt that anyone could have survived in the temple for a hundred years unaided; Aang sarcastically offers the incidence of his own survival as a rationale for the possibility. As Ozai, who was a little unsure about learning air after fire, wonders uncertainly if the person inside can help him understand and teach him about being the Avatar, Sokka offers a hopeful conjecture that the person within may have food, particularly cured meats.

He runs at the doors, briefly becoming irrational in his desire for food, and slams himself into them, failing to open them even after several more tries. Aang bends two focused currents of air into the large doors' ornate locking mechanism, causing the locks to disengage and the doors to open slowly. Ozai slowly walks into the darkened temple as Katara, Zuko, Sokka, and Azula follow him.

Zhao seems appalled by the fact that a submissive bested Bella and her crew, and he refers to the bounty hunter as "more pathetic than [he] thought." Bellatrix furiously states that she underestimated her opponent once, and she tries heatedly to convince Zhao that she will not let it happen again. When Zhao, disregarding the plea, replies that he will be taking over the hunt for the Avatar himself, dismissing Bellatrix as a mere teenager, she leaps at Zhao in rage and has to be restrained by the guards. Zhao orders for Bellatrix and Lu Jing to be kept in his tent and leaves. Bellatrix kicks a table in anger; Lu Jing placidly asks for more tea.

The air temple sanctuary is filled with hundreds of statues. While Sokka, still hungry, is thoroughly disappointed at the lack of food, Katara, Zuko, Azula and Aang notice a pattern in the line of the statues: they are arranged in the Avatar Cycle: air, water, earth, and fire. Putting two and two together, Katara realizes that they are statues of previous incarnations of the Avatar, Ozai's past lives. Ozai singles out two statues in the line and gazes into the first statue's eyes, going into a kind of trance. A gentle shake from Azula breaks him out of it, at which point Ozai recognizes that the statue depicts Avatar Roku, the Fire Nation Avatar immediately preceding him and his grandfather. Sokka recognizes Roku as a firebender by his robes and top-knot, citing this as the reason he did not trust Ozai when they had first met all those years ago.

Suddenly, the six of them hear a humming noise from the entrance, an indication that someone else is nearby; the noise causes the group to take cover behind the statues. Preparing to fight a firebender, Sokka jumps out, trying to catch the intruder off-guard; when Ozai and Zuko peer out from behind their statues, they find themselves looking down on a solitary lemur. Upon this discovery, a slight conflict of interests arises: Sokka, still focused on his hunger, wants to eat the lemur for dinner; Aang, on the other hand, wants to keep the lemur as a pet. This prompts a race to see who can be the first to catch the lemur. Katara stays behind in the temple sanctuary with Azula, Zuko, and Ozai while Aang and Sokka run after the lemur, each trying to trip up the other. The lemur flies out from the window, and Aang, determined to catch the animal, jumps off the temple balcony after it.

Meanwhile, Commander Zhao re-enters his tent, telling Bellatrix and Lu Jing that he and his search party are almost ready to embark on their search for the Avatar. Zhao also says he plans to have his guards release Bella and Lu Jing once his ship has left. Still infuriated, Bella reiterates her belief that she is going to capture the Avatar before Zhao can; the commander laughs at the thought of a bounty hunter competing with himself, who has hundreds of warships at his disposal. Bella, goaded beyond rationality by rage and disgust, challenges Zhao to an Agni Kai at sunset, to which Zhao accepts. After Zhao leaves the tent, telling Bella to prepare to be humiliated, Lu Jing reminds Bella of her last duel with a firebending master, prompting her to say she will "never forget" that fight.

After landing on the ground below, Aang pursues the lemur toward the tattered ruin of a tent. Poking his head inside the tent, Aang, to his horror, finds a number of Fire Nation soldiers' bodies scattered in the snow; walking farther inside, he finds the skeleton of Monk Gyatso. Seeing his former master and father figure's skeleton, he breaks down, falling to his knees in tears. An unfortunately naive Sokka, still searching for the lemur, finally catches up to Aang and playfully asks whether Aang caught the lemur; when he sees Aang on the ground, he goes on to say that he did not actually intend on eating the animal. Only then does he notice the skeletons all around them and understand why Aang is upset. Sokka tries to comfort Aang, but the latter, overcome by emotion, suddenly releases his dominant scent, causing Ozai, who was following Aang and Sokka, to go into the Avatar State out of fear, the sudden glow of Ozai's eyes causing Sokka to gasp and reel away from him in fright. The submissive, overwhelmed with guilt by actions that he had no part of and frightened by Aang's sudden shift into his dominant side, started hyperventilating as sobs wrecked his frame.

Within the temple sanctuary, Katara is looking at the statue of Roku with Zuko and Azula when its eyes suddenly begin to glow white. Continuing to watch in shock as the eyes of all the Avatar statues begin to glow in the same manner, the string of lights spiraling into the darkness above her, Katara realizes Ozai must be the cause, and the trio races out of the sanctuary to find him. In coincidence with the occurrence in the sanctuary, the Avatar temples in the other three nations around the world simultaneously experience similar events, their own Avatar statues and murals glowing spontaneously as well. In the Fire Temple, the Fire Sages prepare to inform the Fire Lord of the return of the Avatar.

Ozai, meanwhile, begins to bend a windstorm around himself, still deep in the Avatar State; Sokka is blown backward off his feet as the gale intensifies, and the tattered tent is obliterated by the blast of wind. Katara, Azula, and Zuko, walking with an arm protecting their faces as they struggle against the wind, ask Sokka, sheltering behind an outcropping of rock, "What happened?" in a yell, and he yells back, "Aang found out that firebenders killed Gyatso! Aunt Ozai was following us, but Aang's dominant scent scared him into the Avatar State. Now, he's having a panic attack while Aang is still mad!" As Katara, realizing that he indeed triggered the Avatar State, resolves to try to calm them down, Aang's rage escalates to the point that the sphere of air containing him begins to float upward, coming to a halt a short distance off the ground, and Katara subsequently begins making her way through the blinding whirlwind toward Aang.

Meanwhile, as the sun sets over the Fire Nation port, Commander Zhao and Lady Bellatrix are in a large courtyard, ready to duel; the two combatants are crouched back-to-back, with some distance between them. Shedding their shoulder garments, they turn to face each other. Lu Jing counsels Bella to remember her basics, as they are her greatest assets, but Bella seems not to heed her brother's wisdom, instead stating, simply and forcefully, "I will not allow myself to lose". As she assumes her stance, Zhao, doing the same, taunts Bella, saying: "This will be over quickly." Seeing the opponents ready for battle, a soldier looking down on the proceedings from the courtyard wall sounds a gong, and the match commences.

The two opponents stare fiercely into each other's unblinking eyes for a brief moment, waiting for the other to strike; it is Bella who begins the duel with a series of fire blasts from her hands and feet as she moves along her side of the courtyard. As the duel progresses, Zhao seems more than a match for Bella, effortlessly avoiding and nullifying all of her fire blasts. As the bounty hunter catches her breath, Lu Jing continues to advise Bella to remember her basics. Zhao throws his own volley of fire blasts; Bella is able to block each attack, but she is slowly forced back with every parry she makes. For the final blast, Zhao, using both fists, sends a ball of fire that connects solidly with Bella, knocking her to the ground. Pressing his advantage, Zhao leaps into the air, covers the distance separating him and Bella, and prepares a finishing blast aimed directly for the bounty hunter's exposed face. An instant before contact, Bella rolls out of the way, rises with a kicking flourish, and knocks Zhao out of his stance.

With new-found vigor, Bellatrix releases a series of low attacks that cause Zhao to retreat, finishing him with a jet of fire from a full body kick. Zhao expects Bellatrix to administer a final, devastating blow to mark her victory, but the latter's fire blast lands beside his face instead; when Zhao insults him for what he perceives to be cowardice, Bella promises that she will not show restraint, should Zhao seek to get in her way in the future. As the victorious bounty hunter walks away, a beaten and furious Zhao sends a jet of fire at Bella's back. Lu Jing intervenes, however, stopping the attack with his bare hand and throwing the commander to the ground. As Lu Jing stands over Zhao, Bella tries to attack Zhao once more, but her brother tells her not to taint her victory by retaliating. Lu Jing lectures Zhao about the dishonor he has brought upon himself through his actions and states that his sister, even on a mission, has proved herself to be more honorable. He politely thanks Zhao for sharing his tea and turns away from him, leaving the defeated commander to pick himself up; he groans as he watches them go. After leaving the arena, Bella asks Lu Jing if he truly meant what he said, to which her brother responds, "Of course. I told you ginseng tea is my favorite." As the two walk back toward the ship, the faintest of smiles crosses Lady Bella's lips.

Shouting over the deafening wind, Katara attempts to console Aang by stating that, "Even though your people are gone, you found a new family: me, Zuko, Azula, Aunt Ozai, and Sokka. We're your family now, but we can't help you if you won't talk to us and let us in!" The words seem to reach Aang; slowly, the wind dies and he descends back to the ground.

Sokka, joining his sister, reinforces her statement with the promise, "None of us will let anything happen to you, or each other. We're family, and we protect our own." Katara takes Aang's hand and he finally calms down, looking visibly sad for a brief moment before collapsing into Katara's arms.

He apologizes for his outburst and Katara tells him that there is no need to be sorry. Aang admits that Katara was right about the firebenders' invasion of the temple and concludes with great sadness that, if one temple was successfully attacked, the others must have fallen as well.

With this, Aang realizes that he truly is the last of the airbenders. Suddenly remembering Ozai, Zuko and Azula rush over to their mother, who was laying unconscious in a snowbank.

Some time later, Aang, Katara, Azula, Zuko, Ozai, and Sokka stand in the sanctuary. Ozai stares at Roku's statue once more; after Azula asks him, "Mom, are you ready to leave?", he wonders aloud to Katara about how a past life, being dead, could help him, to which Azula replies, "Maybe you'll find a way."

Suddenly, the lemur reappears with a variety of fruits and lays them in front of Sokka, who, having been hungry for an extremely long time, wastes no time in devouring them eagerly. The lemur subsequently climbs onto Aang's shoulder.

Later, as Katara and Sokka finish packing behind him, Aang stands overlooking the silent remnants of the deserted temple with Appa and the lemur; he tells them that the three of them are now the temple's only living remnants and that they have to stay together. Calling out to the siblings, Aang introduces the lemur to them as the newest member of their group and Katara, approaching him, asks what its name will be. The lemur promptly leaps off Aang's shoulder, stealing the moon peach Sokka was eating and returning with it to Aang. Fitly, the young Air Nomad names the lemur Momo as he and Katara laugh at Sokka, who, unaware for a moment that his food is gone, takes a bite of nothing but air. While all that was happening, Ozai was curled up in Appa's saddle, resting comfortably.

Later, as night falls, the gang departs from the temple on Appa. His face sad, Aang leans on the pack at the back of Appa's saddle, taking a final, wistful gaze at what was once his home as it is concealed by swirling clouds.