"You know what? I'm really glad you guys all live here now. It's like the hermit crab; maybe you weren't born here, but you found this empty shell and made it your home. And now you protect each other."
— Aang speaking to the Earth Kingdom colonists.
Aang discovers Earth Kingdom civilians have moved into one of the air temples. They are outcasts constantly threatened by the Fire Nation to provide them with new war machines. With help from Aang and his friends, they fend off the Fire Nation and save the temple.
Team Avatar is listening to a story about flying people roaming high in the mountains. Suspecting that the story may be referring to the airbenders, they investigate further. Upon arrival at the Northern Air Temple the next day, they find not airbenders but an Earth Kingdom colony led by an eccentric inventor and his paraplegic son. The inventor has defiled the ancient landmarks with technological "improvements", similar in style to the Industrial Revolution. Aang is at first disheartened by the many changes to the airbending culture of the temple, though he develops a degree of respect for the mechanist's disabled son, Teo. Sokka explores the temple to begin work with the mechanist, helping with advanced ways to detect natural gas leaks through use of rotten eggs. Sokka also aids the mechanist in designing a war balloon.
However, this state of relative happiness does not last. Aang notices that the temple is being destroyed by machines, pipes, and gadgets. Upset and hopeless, he asks Teo if there are any parts of the temple still unchanged. Teo proceeds to show him the entrance to an air temple sanctuary, and asks Aang to open it, stating, "I've always wondered what was like in there." Aang initially refuses, but later opens the gate with airbending only to discover that the mechanist had found or created another way in and was using the room to store monstrous weapons of war, such as self-propelled tanks and war balloons, all destined for use by the Fire Nation.
Later, Aang confronts the mechanist, who confesses his reasons for his actions. He first explains that their village was victim of a terrible flood, which led to the death of Teo's mother as well as his injury. With no place to call home, the villagers fled into the mountains and discovered the Northern Air Temple. The Fire Nation looked to colonize the deserted Air Nomad settlement, which caused the mechanist to plead with War MinisterQin. They later agreed to a compromise where the Earth Kingdom villagers could live in the temple given that they supply an array of weapons to the Fire Nation monthly.
After his speech, the mechanist rushes into his office, followed by Aang and Teo. A bell rings, and the mechanist warns Aang to hide. Aang refuses, and War Minister Qin rises up from a makeshift elevator. He demands his weapons, but Aang appears, closes the door, and traps the minister. Aang tells him that the deal is off and slaps him in the face with an airbending move. Enraged, War Minister Qin states, "The destruction of this temple will be on your head."
Fire Nation forces begin to arrive, using special grappling tanks to climb the steep cliffs. Soldiers march along the mountain paths, wreaking havoc in the colony. The team is in despair over what to do until Aang realizes that they have something the Fire Nation does not have: air power. Sokka and the mechanist quickly plan on how to improve the war balloon and fight back the Fire Nation using it along with the customized gliders. Sokka states that they will use four different types of bombs; smoke, slime, fire, and stink.
The villagers fight courageously, but the Fire Nation tanks overwhelm them. Every time Aang flips the tanks over, the cockpits also flip, enabling them to continue progressing toward the temple. Teo states that the tanks have a water balancing system. This inspires Katara to use waterbending to fight them, but there are too many. Appa arrives, and retrieves Aang and Katara from the battlefield. Out of explosives, the people are losing hope, but the war balloon suddenly appears. The Fire Nation does not attack it, seeing the Fire Nation emblem emblazoned on the balloon. Sokka drops several slime bombs, but that does not stop the advancing army.
As the balloon flies near a large chasm a short distance away from the temple, Sokka and the mechanist smell the pungent odor of the rotten eggs used to detect natural gas leaks. Realizing the balloon is out of bombs, Sokka detaches and throws the balloon's hot air engine into the crack, knowing that the balloon will immediately plummet to the ground without power. The resulting explosion crushes the invaders, while Sokka and the mechanist are rescued by Aang. Deprived of power, the balloon consequently drops into the Fire Nation campgrounds. After the victory, Aang tells Teo that he is no longer upset by the changes made to the air temple; instead he feels happy that they have made the temple their own, and asks them to continue protecting it.
Down the mountain, Qin and his soldiers find the war balloon, saying, "This defeat is the gateway to many victories," as firebenders inflate the balloon.
This episode marks the first appearance of the steampunk elements that will later become a substantial part of this series' story as well as The Legend of Korra's.
When Teo first shows Aang and Katara the door to the Northern Air Temple sanctuary all three tubes on the door are horizontally aligned, but when it shows the door seconds later, the two outer tubes are vertically aligned.
When the mechanist and his men are making room for the bath house, he blows the head off one of the statues of the monks, but when the dust cleared, the head is still there.
When Aang and Teo confront The mechanist in his office, Teo's open-fingered gloves disappear and re-appear several times.
When War Minister Qin and the mechanist are heading for the inventions, Aang closes the door. In a short flash looking through Aang's legs, Qin's hair is loose, but in the next scene his hair is normal, and loose again after Aang uses airbending against him.
When the mechanist is talking to Aang about why he is at the temple, Aang's staff looks like a regular stick.
When the mechanist says, "You're the Avatar? But you're twelve", it was just a few seconds before he met Aang, and nobody mentioned how old he was while with the mechanist, making it impossible for him to know his age.
In "The Southern Air Temple", Aang states that the only way to reach an Air Temple is on a flying bison, yet in this episode the Fire Nation troops are able to walk right up the mountain, and would have reached the temple this way had it not been for the resistance.
Sokka's war balloon, built by the mechanist, is a very unique and important invention. Both of them should have been aware that, if the Fire Nation captured such a balloon, it can spell disaster for future missions. It is surprising that neither took any measures to avoid the loss of the balloon and simply allowed it to crash, whereby it could be recovered by the Fire Nation and reverse-engineered by their scientists.
Some cable TV providers, such as Comcast, list this episode as "The Southern Air Temple", but there is already another episode in the series named as such.
Sokka and the mechanist decide to mix the rotten eggs in with the gas supply so that gas leaks can be detected by the concomitant rotten egg odor. However, eggs that are only a week old—which, the mechanist tells us, is their age—hardly stink noticeably. Perhaps they would stink without refrigeration, but recall that the Northern Air Temple is in a cold geographical area that essentially provides natural refrigeration for perishable foodstuffs.
The natural gas room door has a small sliding door to look into. However, since the natural gas is invisible and the room is dark, there seems to be no point in looking into the room. Furthermore, it might just provide the natural gas another area to leak, albeit small.
The mechanist created candles that spark to tell you the time, but when they were first shown he knew what time it was just by looking at them. The spark powder did not go off until he was explaining it to Sokka. It is possible that he could tell the time by counting the lines on the candles and figuring how long they had been burning, but this would render the spark powder redundant.
When Katara says "I'm sure some parts of the temple are still the same", her mouth didn't move.
When purchased in iTunes, this episode is called "The Northern Air Temple (a.k.a. The Gliders)".
Sokka and the mechanist's idea of adding the scent of rotten eggs to the gas to identify the source of the leak is similar to the real world process of adding ethanethiol to otherwise odorless LPG to make it detectable for humans.
The storyteller at the beginning of the episode bears a striking resemblance to the famous Hong Kong movie director/actor/fight choreographer/ Hung Gar style kung fu master, Lau Kar Leung. He wears an outfit similar to the one Lau Kar Leung wears in the kung fu movie classic, Drunken Fist 2, starring Jackie Chan. Master Leung is well known for the large mole on the right side of his face.
As mentioned by Katara, it takes at least two days for Appa to fly himself and the others from the temple to the Northern Water Tribe capital.