På reise uten sin onkel, vandrer Zuko nå av sted inn i en Jordkongerikeby, hvor han knytter bånd med en lokal gutt. Samtidig blir Zuko hjemsøkt av minner fra sin egen ungdom.

Etter å ha påført seg alvorlige skader i slutten av sesong to, våkner Aang ombord på et Fire Nation-skip. I mellomtiden reiser Zuko hjem.

Four days prior the invasion, Aang becomes nervous over his soon-to-come confrontation with the Fire Lord. His anxiety gets the better of him as he begins to suffer from outrageous nightmares and hallucinations. Meanwhile, Zuko becomes upset to learn that he was not informed about a war meeting.

As the invasion force fights their way to the Fire Nation capital, the kids help Aang find the Fire Lord in time for the eclipse. Zuko confronts his father with shocking news.

Aang struggles to decide whether or not he has to kill the Fire Lord and, during the night, is drawn toward an island which disappears before morning. Zuko leads the rest of the group to the Earth Kingdom to meet a tracker whom they hope can find Aang. The Fire Lord crowns himself Phoenix King and Aang awakens on the island not knowing where he is.

While on the island, Aang seeks guidance from his past lives, but is not pleased with what he is told. The island, in fact, turns out to be a lion turtle from which Aang is given the guidance he had been seeking. After the tracker is unable to track Aang, the rest of the group decide they must find Iroh. Meanwhile, while tracking down Iroh, the group meets up with The Order of the White Lotus, a group that transcends the division of the four nations.

Sokka, Suki, and Toph try to stop the Fire Nation's air fleet. Meanwhile, Zuko and Azula duel in an Agni Kai and Aang confronts the Fire Lord. On the verge of losing, Aang gets hit in the back of his scar which sends him into the Avatar State.

Aang defeats the Fire Lord by bending the elements within the Fire Lord and removing his firebending abilities. After Zuko is taken down protecting Katara from Azula, Katara joins the fight and manages to restrain Azula. Afterwards, Zuko is crowned Fire Lord and promises a better future for the world, while Aang takes his place as the next Avatar. Katara finally shows her love for Aang as they kiss under the setting sun.