Original: The series follows the adventures of the main protagonist Aang and his friends, who must save the world by defeating the evil Fire Lord and ending the destructive war with the Fire Nation.
Simplified: The series follows the adventures of the main protagonist Aang and his friends, Katara, Sokka, and after Toph and Zuko who must save the world by defeating the Fire Lord Ozai and ending the destructive war with the Fire Nation, which started 100 years ago.