Avatar: The Last Airbender: Season 1, Episode 7

1 - 7: The Spirit World (Winter Solstice, Part 1)

When Aang is assigned to help defend a town from a monster that has been plaguing the town, his lack of ability and knowledge becomes obvious, as he doesn't know what to do. However, when Sokka tries to rush to Aang's rescue during the fight, he himself becomes captured and is taken away by the monster, which turns out to be from the spirit world. Aang promises Katara that he'll find Sokka and bring him back, but with Aang even doubting himself, will he be capable of finding where the monster went?