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.

Forvekslet med en Fire Nation-student, blir den kamuflerte Aang tvunget til å registrere seg hos en Fire Nation-skole. Aangs hår hjalp ham å skjule seg bedre med pannebånnet sitt.

Når gjengen kommer til en lidende fiskerlandsby, viser en mystisk ånd seg å hjelpe landsbyboerne.

Når Sokka føler at han ikke bidrar nok i gruppen, oppsøker han en mystisk herre til å lære ham hvordan man bruker et sverd.

Zuko, Azula, Mai og Ty Lee drar på ferie til Lo og Lis strandhus, hvor de lærer om seg selv og hverandre.

Aang og Zuko blir tatt med på parallelle eventyr som gir dem en innsikt inn i deres forfedres fortid - men hvordan betyr fortellingen om Roku og Sozin noe for dem nå?

Katara blir ikke glad når Toph oppdager en rask måte å tjene penger på, og splittelsen dem i mellom får katastrofale konsekvenser.

I en halloweenutgave ser gjengen inn i merkelige forsvinninger i en skummel by. Der lærer Katara hemmelige vannbøyingsteknikker av en gammel dame kalt Hama.

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.

The day of the eclipse has arrived, and the invasion force gathers together to make last preparations. They infiltrate the island and the battle begins. Meanwhile, Aang searches for the Fire Lord.

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 and the group arrive at the Western Air Temple, with Zuko in pursuit. Zuko wishes to help the Avatar end the war by teaching him firebending.

While teaching Aang firebending, Zuko realizes that he has lost all his firebending skills. To regain his powers, Zuko and Aang must seek the original source of firebending within the ruins of a lost civilization.

Sokka wants to break his father out of jail and asks Zuko for information on Fire Nation prisons. Sokka and Zuko head for the Boiling Rock, a prison located in the middle of a volcano, to rescue Sokka's father.

Zuko and Sokka make new plans to break out of the prison with Suki, Sokka's father and some others while a shocking twist leaves Azula none too happy with her two closest friends.

Katara decides to confront the Fire Nation soldier responsible for her mother's death. Aang tries to talk her out of it, but she doesn't listen.

A play recounting the gang's adventures does not sit well with Aang and his friends, except for Toph who enjoys it very much. Meanwhile, Aang confesses his love to Katara, but the response is ambiguous.

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.

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.