Original: He, along with his younger sister Katara, discover an Airbender named Aang, the long-lost Avatar, and accompanies him on his quest to defeat the imperialistic Fire Nation and bring peace to the war-torn nations.
Simplified: He, along with his younger sister Katara, discovers an Airbender named Aang, the long-lost Avatar, and accompanies him on his mission to win the Fire Nation and bring peace to the world.