Upon returning to the Waterbending village, Aang is met with hositlity, as the flare he set off accidentally has lured a Fire Nation ship to their shores. Both Sokka and Kanna agree that Aang must leave the village immediately for what he did. Though Katara fights this at first, Aang convinces her its the right thing to do, so him and Appa leave. After they do though, they eventually see the ship Zuko is on, and become immediately worried for them.
At the South Pole, Sokka prepares for battle, but once Zuko arrives, he's defeated rather easily. Thankfully for them all, Aang arrives once again, and begins to battle with Zuko after its revealed that he is the Avatar. Not long after the battle starts, Aang realizes the whole tribe is in danger from Zuko's attacks, so he offers to surrender as long as the tribe is left alone afterwards. Zuko agrees.
After they leave, Kanna talks to Sokka and Katara, noting that their fates are indeed tied to Aang's, and that they to save him in order for hope to return to the world. Knowing they have to get to him fast, they find Appa, and after using the keywords "Yip-yip!" he begins to fly.
On the ship, after Aang is imprisioned, he waits for his chance, then begins a daring escape attempt. Despite having his hands chained at first, he's still quite powerful, and takes out most of the ship. However, once encountering Zuko again, his task becomes more difficult.
The battle wages throughout the ship, and eventually reaches the top. As Aang tries to escape, Zuko and his forces surround him. Katara and Sokka show up to help though, but Aang is blindsided by Zuko, who knocks him off the ship and into the water.
As Aang begins to sink, his tattoos and eyes glow white, and he begins to Waterbend a powerful waterspout that rockets him back into the air, the then uses his new Waterbending power to incapcitate the ship. But before he can take out the rest of the Firebenders he passes out. Katara comes to his aid though and uses her Waterbending skills to freeze them in place. They get onto Appa and fly away.
Zuko is furious at the loss, and demands that the ship be fixed so they can pursure them. Aang meanwhile thanks Katara and Sokka for their help, and after the two reveal they're staying with him, they make for the North Pole...after a few detours.