Plagued with nightmares featuring the devastating power of his Avatar State, Aang and the gang meet an Earth Kingdom general who has big plans for defeating the Fire Nation. Meanwhile, Zuko and Iroh, having survived the failed attack on the North Water Tribe, have a surprise visit from Zuko's sister with seemingly good news.

On their way to Omashu, Aang, Katara, and Sokka meet a group of nomads who show them a shortcut through the "Cave of Two Lovers". But when the group becomes trapped in the cave, it's up to Aang and Katara to save the group by trusting in love, possibly questioning their relationship. Meanwhile, Iroh gets a lethal rash so Zuko and Iroh head to a small Earth Kingdom town for help, hoping their real identities will not be discovered.

Despite the shocking news the gang have just discovered, Aang insists on trying to find King Bumi within Omashu's borders, even with the dangers that will await him and his friends if his real identity is discovered. Meanwhile Azula goes in search of old friends to help her with her mission.

A mysterious tornado pulls the kids into a supernatural swamp where Katara, Sokka and Aang have mysterious visions. Meanwhile, Momo and Appa are chased by rogue swamp dwellers and only Aang and the gang can save them.

Aang and the gang discover an Earth town that celebrates something called "Avatar Day." But when they get there and learn the festival is of "anti-Avatar" sentiment and Aang is put on trial for a crime a past incarnation allegedly committed. Looking to clear the Avatar's good name, Katara and Sokka venture back to a familiar place to gather evidence and discover the truth.

In search for an Earthbending teacher for Aang, the kids go to an underground Earthbending tournament called Earth Rumble 6, where they encounter Toph, a blind but powerful Earthbending girl. After seeing her unique style of Earthbending, Aang wants Toph to become his teacher, but there are complications with Toph's family.

Zuko travels by himself to an Earth Kingdom town were he meets a local boy. The two almost become friends, while Zuko suffers from flashbacks of his past.

Tired from trying to escape a machine that shows up every time they try to sleep, Aang and the gang have to escape Azula and her team, but their exhaustion leads to a tight situation. Meanwhile, Zuko must deal with something personal, and Iroh bonds with an unlikely individual.

Toph's instructing abilities come into question when Aang cannot seem to overcome a block that keeps him from earthbending. Sokka gets himself in a jam, while befriending what was, moments before, his next meal, and Iroh and Zuko begin training utilizing new philosophies and methods that originate from the other elements.

When Aang and his friends find a professor who knows of a library that contains information about the world, Sokka decides to go there to hopefully find some information to use against the Fire Nation. The library is guarded by the knowledge spirit, Wan Shi Tong, who doesn't welcome humans into his library, because he believes they only seek information for evil purposes. When Wan Shi Tong discovers Sokka wasn't being truthful with him, he begins to sink the library, bringing the kids down with it.

Aang searches for Appa while Katara struggles to keep everyone together to survive in the vast desert. Meanwhile, Iroh and Zuko seek help from an ancient secret society.

When Aang and the gang meet up with a group of refugees, they tell the gang where they can get on a ferry to Ba Sing Se. But after a series of events they have to take a deadly route called the serpent's pass with the help of an old friend, Suki. Meanwhile Zuko and Iroh meet up with a boy named Jet and his freedom fighters.

Having made it to the outer wall of Ba Sing Se, the kids confront a Fire Nation army that is driving a massive Fire Nation drill, which poses an imminent threat to the last Earth Kingdom stronghold. Meanwhile, Zuko and Iroh make crucial mistakes which begin to attract Jet's suspicions.

Having finally made it into Ba Sing Se, the kids are given a house in the upper ring. The kids are anxious to tell the Earth King their plan but there are complications. Meanwhile, still convinced that Iroh and Zuko may be Fire Nation, Jet decides to take matters into his own hands and do whatever it takes to get the refugees to reveal their true nature.

Appa's journey from being abducted in the desert to the present is explored.

Aang and his friends continue to search for Appa and, unbeknownst to them, they aren't the only ones looking for him. During their search, the kids meet up with Jet, whom they are understandably reluctant to trust. The kids soon realize that the Dai Li are doing their best to thwart the kids' efforts to find Appa. Meanwhile, Iroh is offered his own tea shop in the upper ring, and Zuko discovers the avatar is in the city.

The kids battle Long Feng and the Dai Li when they attempt to show the Earth King the vast conspiracy taking place in his city. Zuko becomes seriously ill and has feverish dreams as he faces a spiritual crisis.

Aang travels to the Eastern Air temple to receive guidance from Guru Pathik while Sokka is reunited with his long-lost father. Meanwhile, Azula has something diabolical in store for her enemies in the Earth Kingdom, including Zuko and Iroh.

Everyone races against time to stop Azula's sinister plan. Zuko finds companionship in an unlikely source. Aang's destiny hangs in the balance—and so does Zuko's, as Iroh tells him it's time to make a choice.