|wow, okay... Aang goes to the spirit world so that the previous avatar, Roku, could tell him the story of how the fire nation became an enemy..|
Roku was friends with the Zouzen, the prince of the fire nation, before he found out he was the avatar. After he went around the world to learn air, water, and earth bending, he returned to his friend who was now the fire lord... and at that point, Zouzen started thinking that the fire nation's prosperity should be expanded and spread around the world, but Roku warned him against this and said that the four nations must be in balance.
...Meanwhile, Zukko gets a message saying that he should read about the fire nation's secret history to learn about when his great grandfather started the war. He sneaked passed the fire sages into the temple and read about the same story Roku is telling Aang, but from Zouzen's perspective...
Roku kept Zouzen from carrying out his plan until they both became old, in the end, Roku died from a volcano eruption, and Zouzen decided not to rescue him... Zukko doesn't learn anything new from this, and wonders why his uncle sent him this message. Then his uncle drops the biggest twist yet: Roku was Zukko's great grandfather on his mother's side.
Evil and good are always at war inside you, Zukko. What happened generations ago can be resolved now, by you.
He gives Zukko the prince's crown, the same one that Zouzen gave Roku.
also the guy that Zukko hired to kill Aang has a 3rd eye on his forehead and can make things explode from a distance... Aang's group can barely even defend against him when they're working together.