I’m watching Avatar: The Last Airbender 3×13 “Chapter Thirteen: The Firebending Masters” https://trakt.tv/shows/avatar-the-last-airbender/seasons/3/episodes/13
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.