"I'll never forgive him. But I am ready to forgive you."
Our heroes are woken explosively by the arrival of a fleet of Fire Nation airships, with Azula at their head. Zuko buys time by facing her one-on-one, completing his integration into The Gaang, who now seem to have no further reservations about his presence; he even jokes about chasing Aang around for old time's sake. Only Katara continues to resist, explaining that she can't forgive him and the Fire Nation for all they've done.
That night, Zuko goes to Sokka for advice and is treated to a Flashback
of how their mother Kya was killed during a Fire Nation attack on the Southern Water Tribe. The next morning, Zuko offers to help Katara hunt down those who slew her mother. Sokka and Aang are hesitant, the Avatar in particular stressing that forgiveness and not violence will bring peace, but when Aang discovers Katara preparing to steal Appa if necessary, he relents and lets them go.
Using Zuko's knowledge of Fire Nation communication channels, they identify the division which was responsible for Kya's murder, The Southern Raiders. From there, they track down the group's retired captain, Yon Rha, who now lives with his overbearing mother. Upon being confronted with this vengeful Waterbender, he expresses incredulity: he was dispatched to the Southern Water Tribe to kill their only living Waterbender, which Kya claimed to be. Katara prepares the killing blow... only to stop at the last moment, deciding that this shriveled, pitiful man doesn't even deserve death.
Back at the camp, she hugs Zuko and thanks him, seemingly over her mis-directed hatred. Aang comes, saying how proud he is that she was able to forgive. She shakes her head, saying coldly "I didn't forgive him. I'll never forgive him." Nonetheless, Aang is still proud, and stresses that violence is never the answer.
Zuko has only one answer to that: what is Aang going to do when it's time to face the Fire Lord?