Katara, in "The Puppetmaster", found a dark side of water bending: Killing plants and bloodbending. There seems to be no problem with getting water from the air, however, that takes a while and doesn't give much water.

Zuko has gone through the dark side of firebending, lost his firebending, then found the light side of firebending in TFM.

There has been a lot of emphasis on the fact that Sokka and Katara's mother was killed by the FN, but we haven't seen any flashback about it. Also, we haven't seen any reaction from them that shows that they've ever met up with the firebender that killed their mother.

In "The Boiling Rock" we know Zuko will get captured and it's likely he'll get caught to ensure that others get away. This is the foot-in-the-door that gets Katara to stop hating him, at least as much. Remember, he's pretty much at a point where her opinion of him really has no where to go but up. EDIT: Also, when Zuko is in his cell and Mai comes to see him, it'll probably be there that the two break up but stay friends at some level. This will also be where Mai does her good deed for the series, helping the Gaang free Zuko even though he isn't her boyfriend any more. After all, Mai isn't evil. She just helped Azula because she was bored.

Sooner or later, after "The Boiling Rock" the Gaang will come across, among others, the firebender responsible for Sokka and Katara's mom's death. However, no matter how much Sokka and especially Katara want to fight him, with the avatar and Zuko there, he isn't going to be too interested in taking on a couple of Water Tribe peasants. This gets Katara more enraged and, as she attacks him, she runs out of her supply of water that she carries/is present. Because of that and the fact that she wants revenge, she starts drawing water out of the plants, killing them, and constantly getting the water evaporated.

The fight splits up with the firebender taking on Zuko and Katara while the rest of whatever force he's with handles the others. Eventually, the fights get so far apart that the rest of the Gaang can't hope to help Zuko and Katara.

Now, Katara is killing the plants left and right, thinking of nothing but revenge, and acting recklessly while Zuko is trying to fight smart, protect Katara from getting hurt because of her recklessness, and holding back his attacks when she jumps in front of them so that she can kill him. Well, sooner or later, Katara stops being able to bend; the water literally stops obeying her. Just then, the firebender attacks her to get her out of his way and Zuko has to take the bullet while firing his own to temperarily stun the firebender. The "bullet" causes Zuko to be so injured, he can't move (maybe it's another lightening user and Zuko doesn't have time to redirect it, it'd make sense if he were for later in this).

Shocked that Zuko risked his life for her, she sees if he's ok and he explains to her how he knows what she's going through because it's similar to how he used to feel about Aang. He then tells her that she's destroying the memory of her mother and everything her mother taught her by waterbending the way she is. He explains that, she's now fighting solely for revenge instead of to protect people, which, as emphasized throughout the series, is a driving force for her. Throw in a little extra dialogue about this or that, a few tears, and Katara is back up, ready to waterbend for the right reason. As said, that reason is to protect people who need her, which includes Zuko.

She wins the fight and thinks the firebender is dead or at least unconscious. She begins healing Zuko and the two begin a little discussion how she's sorry for everything and that he forgives her, yada, yada, yada. Well, the firebender get's up and starts building up lightening. Katara doesn't see the lightening since she's healing Zuko, but he does see it. As soon as the lightening is fired, Zuko gets a sudden burst of strength, pushes Katara out of the way, redirects the lightening back at him, and kills him. Once the smoke clears, the burst leaves Zuko and he collapses on the ground with Katara screaming his name out of fear for him.

After the commercial break that would happen there, Zuko wakes up with the group standing over him. They all express their gratitude for what he did and how grateful they are that he is alive. Katara then asks for a few minutes to talk to Zuko alone, saying that she has something to say to him.

Once they leave, Katara begins shyly and thanks him for saving her. Once they establish the obvious, that "saving her" is about more than the lightening at the end, she says that she's sorry for everything she's said to him and, despite how much hey may have deserved her suspicion at first, it should have been obvious that he really had changed this time. The two share a small polite talk and Katara hugs him and says thank you. When she does this, she touches a bruised spot on him and he winces in pain. She instantly pulls back and nervously apologizes. Zuko says that it's ok and, when they look at each other to continue their talk, their eyes meet, there's one of those moments, they lean in, and kiss.

Meanwhile, Aang either remembers something he needed to ask Katara or she dropped something during the fight and all the concern about Zuko made him forget to give it to/ask her. He goes back to where she and Zuko are and sees them kissing. I don't know if they'll see him, but, I do know that, once he sees them, he'll get that "I don't know what to do" expression on his face and run off.

End episode, end shipping wars, and begin Zuko and Katara's relationship.

For the record, unless you want to respond to a specific part of this, please reply with @daveshanFromGMU or "To daveshanFromGMU". This is a huge block of text and we don't need it quoted every time someone wants to reply to it.