After Azula, Mai and Ty lee pretended to be Kyoshi warriors. They sent Katara and Zuko to the catacombs under Ba Sing Se. As they're stuck together under there, you can just sense all the chemistry. Katara was angry, while Zuko was sad. He had figured out his path after he set Appa free from the prison of Ba Sing Se. Zuko started to talk to Katara, but she projected all her anger at him.
"What is this a trap! Just so you can have Aang and your stupid honor in your dirty hands?!" Katara says angrily. "No, my stupid sister and her friends sent me down here. We're both on the same page" Zuko says calmly. Katara can finally see the good inside of him. "You know, when I thought of the face of the enemy, it was always you." Katara says. "My face. I see." Zuko says sadly touching his scar. "No. I didn't mean it that way. It's just the fire nation took my mother away from me" Katara says touching her necklace. "That's something we have in common" Zuko says.
Katara turns her head in shock and comes closer. The tension is just filling the catacombs. Katara offers the spirit water from the northern water tribe so she can heal the scar so he could no longer live under the fear and disappointment his father put upon him. Zuko refuses "It's a scar! It can't be healed" as he turns away, Katara comes so close that their clothes are rubbing together. She touches his face and Zuko slowly closes his eyes and grabs her waist. He starts to lean in for a kiss, Katara leans in too.
*BOOM*
They blush as they quickly lean and turn away from each other. Aang and Uncle Iroh storm in. Aang is completely clueless to what was going on but Uncle Iroh catches the feeling. Katara forces herself to run up and hug Aang even though she just wanted to hug Zuko and never let go.
Azula also storms in with members of the Dai Li as they capture Uncle Iroh. Thankfully Katara and Aang had already escaped the catacombs. Azula gives Zuko a hard ultimatum. Uncle iroh and a life of treachery in her words or their fathers love. Even though he had finally found his path, he was confused and overall he did a long journey just to regain his honor and his fathers love. Uncle Iroh was so sad and begged Zuko because he never wanted him to lose his way
Zuko chose Azula, and together they battled Aang and Katara. As Aang entered the avatar state Azula hit him in the spine with a jolt of lightning that killed him. Katara revived him of course. But she was so terribly sad because of Zuko. Even though they were together for such a short amount of time she was the first one to trust him.
Zuko regretted it so badly but loved the feeling of knowing his father would respect him again, but all he could think of was Katara and how he hurt her. He saw the pain in her eyes when he battled her once again. He felt so much more pain when Mai arrived and kissed him. All he wanted was Katara but he didn't know it yet.