Katara:
[Aang stays behind as the citizens leave Omashu]
Aang? What are you doing? Aren't you coming with us?
Aang:
No. I'm not leaving until I find Bumi.
This quote occurs at 00:09:08-00:09:16 in the video.
Finding the city of Aang's old friend, King Bumi, captured by the Fire Nation, the heroes hatch a plan to rescue Aang's earthbending teacher. Azula gathers two old friends to help her hunt the Avatar.