Disclaimer: All rights belong to Nickelodeon, Bryan Konietzko, Michael Dante DiMartino, and all the men and women that created the A:TLA show, books, and comics. I take no credit, and I do not mean to break any copyright rules. This is simply a work of fiction made for enjoyment. No money is being made. The lyrics are from the song "Soldier" by Fleurie
Rating: General Audiences. Warning: some scenes contain dark themes and minor violence
Chapter 12: Rescue
Soldier keep on marchin' on
Head down 'til the work is done
Waiting on that morning sun
Soldier keep on marchin' on
They fly overnight. Aang can't sleep due to his anxiousness, so instead he sits atop Appa's neck and watches the stars overhead. Part of him is excited to see the legendary city of Ba Sing Se, but mostly he's just excited to see Katara again. She hasn't left his mind since she was taken.
He wonders if Zuko is still with her. If he is, what will he do when he's rescued? Will he join them? Aang isn't sure how he feels about that. As nice as it would be to have a Firebender on their side, Aang would rather someone else. He knows he's selfish to think that, but he can't help himself.
The others sleep. Sokka is curled up, his backpack in his arms and Momo against his back. Toph is, like normal, by herself on the opposite side. She sleeps restlessly like she always does when they're flying.
Appa is tired, but he understands the urgency. And Aang promised to let him sleep for as long as he wants once they get to Ba Sing Se.
To keep his mind off Katara, Aang tries to imagine how Azula will try to conquer Ba Sing Se. The last time the Fire Nation tried to conquer the city, they surrounded it and imparted a siege. No food, supplies, or people in or out of the city for months. But they underestimated Ba Sing Se's ability to provide for its citizens by itself. There is enough space within the walls to farm food for the city, and in the winter they had grain storage and livestock. When it was obvious that a siege wouldn't work, they tried storming the walls. But the city has an army of Earthbenders that stood atop the tall ramparts and deflected every fireball and every other form of attack. The walls were too tall to try to climb.
Ba Sing Se is simply an impenetrable city.
But Aang knows that Azula is smart and has all the resources of the Fire Nation behind her. He thinks of the attack at the Air Temple. Those all-terrain tanks had almost proved to be their undoing. And he can't even begin to imagine what other nasty surprises the Fire Nation inventors and engineers have come up with.
He finally gives up and tries to get some rest, although his mind never completely shuts down.
Dawn is just breaking on the horizon when the imposing walls of Ba Sing Se come into sight. Aang grabs his glider and takes off to do some scouting, Momo right on his heels. Sokka and Toph are just getting up and ready for the day.
Sunrise is the best time of the day to fly in Aang's opinion. Not only is the sky beautiful, but the air is cool and fresh. It always helps him to reset for the day.
He and Momo fly to the wall and then pull up hard, flying vertically upwards along the stone. There's not a single scratch or crack. Never has a wall been so well designed.
They fly a bit over the top and then land on the walkway. Aang looks inward to the farmlands. Then he and Momo head to the parapet and look out.
A loud noise suddenly breaks through the morning stillness. The entire ground, including all the way up the wall, begins to shake. Aang lifts a hand to his forehead to block out the sunlight. He is instantly blinded by a flash; when the spots clear, he sees the source of the disruption: a dozen of the Fire Nation all-terrain vehicles are rumbling forward towards the wall. And even worse - a huge metal drill is rolling slowly forward, flanked by the tanks.
All of them heading straight for the wall of Ba Sing Se.
"This tank is a feat of scientific ingenuity and raw destructive power."
The Fire Nation general struts along the cockpit, his hands folded behind his back, his chin in the air. Azula is sitting on a throne behind him, facing straight forward towards the wall. Ty Lee is perched excitedly on the edge of her seat on the left of Azula. On the right, Mai is sitting with her back to one armrest and her legs thrown carelessly across the other. She's picking at the tips of her hair.
Katara is chained to the console facing the girls. If she twists she can see out of the window, but she doesn't want to attract the attention of the princess.
"Once we tunnel through the wall, our troops will storm into the city," the general continues. "Ba Sing Se will finally fall and you can claim it in the name of your father. Nothing will stop us."
"What about those muscle-y guys down there?" Ty Lee asks, pointing straight ahead. Katara finally does stretch up and twist, getting a good look at the men the girl is referring to. In deep trenches in front of the outer wall, dozens of Earthbenders are settling in for the defense.
"Please," the general says, flicking a metal viewer and causing Ty Lee to jump backwards in her chair. "The drill's metal shell is impervious to Earthbending attacks."
"Oh, I'm sure it is, War Minister Ching. But just to be on the safe side -" Azula sits up and hardens her voice. "Mai and Ty Lee, take the Earthbenders out."
Mai twirls her metal blades around her fingers, catches them in her fist, and stands up. "Finally, something to do," she grumbles. Ty Lee jumps up and follows enthusiastically. Katara feels sorry for the poor Earthbenders. They have no idea what's about to hit them.
Sokka is just tying his bag shut when Aang and Momo land back on Appa's back.
"Have a good morning flight, Twinkle Toes?" Toph asks.
"Actually, no." Aang snaps his glider shut. "I found Azula."
"And Katara?"
"I didn't see her, but I bet I know where she is." Aang glance out. "You'll be able to see for yourself once we get out of this canyon.
The words have just left Aang's mouth when the canyon opens up and the walls of Ba Sing Se come into sight in their full glory. Sokka only has a moment to express awe over it before the drill comes into sight.
"Please tell me that's not Azula," Sokka begs.
"It's Fire Nation, so chances are it's Azula. And I bet Katara is inside there somewhere. Appa, take us to the wall." Aang paces the saddle. "I hope Suki and the Kyoshi warriors are here soon."
"I've always heard stories about the legendary Earthbenders from Ba Sing Se," Toph says. She pounds a fist into her palm. "Let's see how well they hold up in person."
"I don't know if even the best Earthbenders can hold their own against Azula," Sokka says worriedly.
Appa lands on the wall and they all jump off. "Wait here," Aang tells his bison.
A group of Earthbenders run over to them. "Hey!" one calls out. "What are you doing here? Civilians aren't allowed on the wall!"
Aang steps forward. "We're companions of the Avatar," he says. "We need to speak to your leader."
They're escorted to a building build onto the wall. An Earthbending general sits atop a stone chair and greets them.
"Welcome, friends of the Avatar," he says. "We're honored to have you here."
Sokka can't believe this man can sit and smile as if they're about to have tea when there's a thundering metal drill headed straight towards the city.
"We're honored to be here," he says quickly. "We've come to help stop the drill."
The general smiles. "Ah, we appreciate the offer. But it's not necessary. I have the situation completely under control." He stands and stretches a hand out. "I assure you, the Fire Nation cannot penetrate this wall." He leads them out and onto the parapet, where they have a great view of the drill. "Many have tried to break through it, but none have succeeded. That's why they call it Ba Sing Se. The Impenetrable City." He smiles. "They don't call it 'Na Sing Se'."
Toph crosses her arms across her chest. "Yeah, thanks for the tour. But we still got two problems."
"Two?"
"The drill. And our friend."
"Don't worry," the general assures them. "I've set an entire group of Earthbenders to stop the drill. Called the 'Terra Team'. They'll stop the drill and retrieve the Avatar from the Fire Nation."
"Terra Team…" Sokka muses. "That's a good group name. Very catchy."
Toph smacks him.
They watch in silence as the Terra Team moves together, perfectly in sync, to earthbend a large chunk of rock forward. It knocks one of the tanks into the air. Then the Earthbenders lift the earth they're standing on and move toward, towards the drill. The other groups do the same, surrounding the drill and throwing large wedges of rock into the sides of the machine.
For a split second, the drill slows. Then it continues forward, the rock wedges breaking away. The Earthbenders are left on the defensive as a rain of blades falls towards them. Out of the dust appear two familiar faces.
"Ty Lee and Mai," Sokka curses under his breath.
Ty Lee jumps and flips around the Earthbenders, avoiding their attacks and quickly chi-blocking them. They fall to the ground one after another. Within a minute's time, all the members of the Terra team lie motionless on the ground.
"We're doomed!" the Earth Kingdom general cries.
Sokka smacks him. "Get ahold of yourself, man!"
The general rubs the spot where Sokka smacked him. "You're right. I'm sorry."
Toph stands with a placid expression on her face. "Maybe you'd like our help now?" she asks suggestively.
The general bows his head in shame. "Yes, please," he says in a small voice.
Sokka motions for them to follow him back to the building, where he unrolls a map of the outer walls on the desk. He takes a chalk pencil and lays it out in front of the wall.
"This is the drill," he explains. "It's safe to assume that Azula and Katara are inside. We know already that Ty Lee and Mai are outside. We have to stop the drill and rescue Katara." He looks up at the general, Toph, and Aang. "That means we have to split up. One party needs to draw out and distract Azula and her friends, one party needs to focus on stopping the tank, and the third needs to rescue Katara."
"How are we going to do all that? You saw how easily Mai and Ty Lee took out the strongest Earthbenders," Aang says.
"That's because the Earthbenders don't know them. They underestimated them. But we know how powerful they are. Aang and Toph, you guys are going to have to distract Azula, Mai and Ty Lee. I'm going to go in and rescue Katara. Then we can regroup after and make a plan to stop the drill."
"You're tough, Sokka, but not enough to rescue Katara on your own."
"He won't be alone."
They all turn to see Suki walking towards them, once again in her Kyoshi Warrior get-up. The other warriors are behind her.
Sokka's mood instantly brightens. He actually thinks they have a chance at pulling this off now.
"Now you show up," he says sarcastically. "Alright, small change of plans. Aang and Toph, you have the same goal - but now you'll have some backup." He turns to the general. "Are there any other Earthbenders left?"
The general nods. "Not many, but a few."
"Good. Send them with Aang and Toph. The Kyoshi Warriors, too. Suki and I will sneak into the drill and find Katara."
Aang opens his mouth to protest, but Sokka already knows what the kid is going to say.
"Azula will expect you. You're powerful and loyal to Katara. It's better that Suki and I go in - we're not benders, so they won't notice our absence immediately, and with only two people we can sneak in more easily."
Aang nods.
"Alright. Everyone know what they're doing?" Sokka looks around to the assorted "army". They all nod. "Good. Then let's go."
"Alright," Toph says when they're all on the ground. "I'm going to throw up some dust to give us cover. You won't be able to see, so stick close to me."
She waves her arms and stirs up to earth ahead of them. As promised, the wall of dust is so thick they can't see anything through, much less the drill. But they trust Toph's sense of direction and run after her.
"Hey," Ty Lee says, staring into the viewer. "Look at that dust cloud that just suddenly appeared. It's so...poofy." She makes a fist and then spreads her fingers. "Poof."
Azula looks over at her suspiciously.
"Don't worry, Princess," the minister of war says. "I'm sure it's nothing."
Azula isn't so convinced.
"Here." Toph makes a hole in the ground. "Sokka, Suki, in. The rest of your wait here. I'll be right back."
They jump into the hole. The outside closes behind them. They make their way through the tunnel by feel.
"It's so dark in here," Sokka complains.
"Oooh, what a nightmare," Toph retorts.
"Oh. Sorry."
They emerge underneath the drill. Suki spots the opening immediately. "There! Where the red light is!"
Toph uses a burst of earth to launch them up. Sokka and Suki each grab ahold of the pipes and swing themselves up. Then they peer down.
"We'll see you soon!" Sokka calls out.
Toph nods and then tunnels back out, disappearing without a sound.
As soon as Toph reappears, Aang grips his glider tightly in his hands.
"Time to make a distraction," he says. He twirls the staff, dissipating the dust. Then he leaps up and flies by the cockpit, waving to Azula and the other Fire Nation soldiers inside. A second later the entire drill lurches slightly to the side. Down on the ground, Toph and the few remaining Earthbenders are launching a vicious attack on the outside shell of the drill.
Aang flies past the control window one more time and isn't surprised to notice the clear absence of the princess and her cohorts. What does surprise him, though, is the fireball that whizzes just past his ear a second later.
Azula is standing on top of the drill, a blue ball of fire in between her hands. She grins cruelly at him.
"I was waiting for you to show up," she shouts. "I'm only disappointed it took you so long."
Aang flies down to face her on the other side of the drill. As he lands, he takes a quick glimpse down to the ground, where Toph, the Kyoshi Warriors, and the Earthbenders are battling Mai and Ty Lee. All is going according to plan here on the outside; he just hopes that Sokka and Suki are also working according to plan.
Aang dodges another fireball and sends a blast of air towards Azula. She slides backwards a few feet but retains her footing. She looks up and him and he can see the fire burning behind her eyes. It scares him.
She flings out her arm and a whip of fire licks the air where his legs were just a half second ago. But he's already leaping in the air, flipping to the side and kicking out another blast of air. Azula dispels it with a wave of fire.
"You're not bad," she says. "I've always enjoyed a good challenge."
She sends a rapid succession of fire attack his way that he just barely dodges.
"Of course, I'll still have to destroy you."
Aang falls on the defensive and sincerely hopes he can hold her off long enough for Katara to get rescued.
Mai is, of course, a threat, but at the moment Ty Lee is doing the most damage. Every time she does a Chi block she takes down one more bender. And all she needs is the slightest of openings to make her signature move.
Therefore Toph has devoted her energy and focus to keeping the acrobat on her feet. Toph follows the girl's light footsteps and sends a rock pillar to blast her up into the air every time she gets close to a bender or Kyoshi Warrior. Of course, with the footsteps of so many people and the vibrations of the drill and the tanks, it's hard to find that particular set of light footsteps. And the girl is so fast that sometimes Toph reacts a microsecond too late.
Despite Toph's best efforts, Ty Lee is still taking out too many warriors too fast.
The Kyoshi Warriors and other benders are focused on Mai, who is whipping out her ninja stars as fast as one can blink. She fights ten opponents at once with apparent ease. Toph wants to blast her into the air, but she's too preoccupied with the other girl.
Then an opportunity opens up that pleases Toph greatly. Killing two birds with one stone. Or rather, knocking out two girls with one rock.
Ty Lee takes a step exactly where Toph predicts she will. The Earthbender sends a rock blast from an angle that shoots her sideways, right into Mai. The two girls collide and hit the ground hard. They climb back onto their feet within seconds, but that's all the time the others need to regroup again.
Both girls narrow their eyes at Toph. She can't see it of course, but from the way they position their feet facing her, she knows she's become a target.
Good, she thinks with a smile. Let's see what you've got.
Mai opens her fist and three blades appear. "Where's the other one?" she asks. "Your pet nonbender. I understand why you bring him along - his sarcasm is so entertaining."
She throws her blades. Toph erects a wall of rock as a shield in front of her. Then she drops it back down into the earth.
"And I see why Azula keeps you around - your sarcasm is also so amusing." Toph moves slightly to the side as Ty Lee somersaults right behind her. Then she sends a rock formation to slam into the girl, sending her sprawling on the ground.
"You're not even trying to stop us," Mai accuses. "You're just taunting us."
Then she sucks in a breath suddenly. "You're just distracting us."
She turns and begins to run back towards the drill. Toph raises her arms to blast her into the air when suddenly her legs are knocked out from under her. She lands on her back and the breath is expelled from her lungs.
She can feel two sets of footprints sprinting away. Toph curses under her breath and climbs to her feet. At least she didn't get Chi-blocked. She just hopes Sokka and Suki are fast enough because they're about to get some company.
Sokka and Suki find themselves in an engine room. It's dimly lit by red lighting. They walk along a steel gangway, pipes and gears surrounding them.
"I need to find a schematic," Sokka says. "Katara is most likely in the control room, or the cockpit, but I have no idea how to get there. This drill is massive - it could take us forever to find her."
"A mechanic would have schematics, right?" Suki asks.
"Probably. But where do we find one?"
"We don't." Suki pulls out her metal fan. "They'll find us."
She slices down on a nearby pipe, cutting straight through the metal. Steam bursts out with a hiss. An alarm sounds, echoing in the metal tunnel.
"What are you doing?" Sokka cries. Suki uses her hand to cover his mouth.
"Wait," she whispers, leading him a few steps back.
They don't have to wait long. A mechanic runs over. He's so focused on the burst pipe that he doesn't notice them.
Sokka understands immediately. He pulls out his boomerang and throws it. It hits his target perfectly and the man slumps unconscious to the ground. They converge on him instantly, searching his pockets for a blueprint of the drill.
"Here!" Suki holds up the paper. "I got it."
They unroll it and scan it. "There should be a ladder just down the hall and to the left," Sokka says, following the path with his finger. "Then we can just climb all the way to the top."
Suki stuffs the paper in a pocket and they take off down the hall. They scale the ladder. As they pass through the other levels and see the inner workings of the drill, Sokka gets an idea of how to stop it. But first, they'll need to save Katara.
They ascend to the very top, where a door above them is the only obstacle between them and Katara. Sokka twists the circle and the door opens.
"Ready?" he asks.
"Ready."
Suki jumps up first, her fan brandished to deflect an oncoming fire attack. Sokka swings up after her, his boomerang already in motion. They make quick work of the few Fire Nation soldiers inside. When it's quiet again, Sokka sees Katara chained to the wall. He instantly starts searching the guards for the key. Suki heads towards the control console.
"The drill is still advancing," Suki says while Sokka frees Katara. "It must be on some form of automatic steering."
Sokka hugs Katara tightly. Suki steps back long enough to greet Katara.
"I'd love to stay and talk, but we need to save Ba Sing Se. Then we can catch up," the warrior says after a quick embrace.
"I think I have an idea," Sokka says. "When we climbed up, I noticed the support beams inside. We were trying to destroy the drill from the outside, but the metal shell is too thick. What if we sabotage it from the inside?"
"That's a good idea. But how are we going to cut through the beams?"
"Maybe Katara can with her waterbending."
Silence. Sokka turns around. "Katara?"
But she's gone. He turns and trades confused looks with Suki. She must have disappeared already, off to join the fight.
Sokka sighs. "Alright. We gotta come up with a new plan."
"Let's stick to the old one," Suki suggests. "We freed Katara, so now we meet up with the others to make up a strategy to stop the drill."
"You're right. That the best thing to do right now."
They start climbing down the ladder again. They make it as far as the level with all the beams when suddenly a blade whips through the air and pins Sokka to the wall by his shirt. Suki is able to deflect the second one that comes her way.
Out of the steam, two figures appear. One has her hair up in two buns and one has a long braid.
Toph feels a heavy sense of dread as she steps into the drill, but she has no choice - she failed to distract Mai and Ty Lee long enough for Sokka and Suki and Katara to escape. Now she has to track the girls down and help the others.
Although she's willing to finish her job, she doesn't have to like it. She hates how cold and hard the metal under her feet is. She already misses the coarse familiarness of the earth beneath her. She knows her bending powers are useless in this metal trap. She hasn't felt so useless in a long time.
Ty Lee and Mai aren't quiet as they make their way through the drill. Toph follows the sounds of their echoes. She's surprised to find that she can still feel the vibrations of their footsteps through the metal. At least she's not completely blind in here.
The footsteps leave the floor and then the vibrations start coming from the walls. It takes Toph a minute to realize what happened: the girls must have started to climb a ladder somewhere.
She curses under breath and starts feeling her hands along the wall, trying to find the ladder among the pipes. A few times she scorches her hand on a burning pipe but she keeps going. She has to.
She finally finds the ladder and begins to ascend slowly. On the second floor, she can hear the sounds of fighting coming from the walkway. She can feel the clang of a body hitting the ground. She hopes it's not one of her friends and starts running towards the skirmish.
She hears Sokka yell "Ha!". She hears Ty Lee's giggle. The sound echoes through the metal tube, bouncing off the walls and coming back. This is exactly what Toph feared: the confusing incoming sensory signals are leaving her worse than blind.
She's totally useless here.
She hears Suki cry out and hears a sickening thud. A strong vibration runs through the metal beneath Toph's feet. A second later, she feels light footsteps land behind her. Toph whirls to avoid Ty Lee's attack. The acrobat's fists slam into her shoulder. Toph grunts.
"You're out of your element, aren't ya?" Ty Lee asks. Toph narrows her eyes and ducks to avoid getting hit in the face. Then she punches out herself. Her fist connects with flesh, but she's not sure exactly what part of Ty Lee she hit. She's just swinging in the darkness right now.
She hears Sokka grunt and feels another bang and wave of vibrations carry through the metal. She knows she's the last one fighting now.
"Just give it up," Mai says. "You may be one of the best Earthbenders in the world, but you can't bend metal."
Bending metal...Toph places her hands on the railing behind her and focuses on it. Metal has helped her to see the same way the earth has: through vibrations. And what is metal, really? Metal is an element, just like water or fire or air or earth. No - metal is earth. Metal comes from the earth. It's just been refined.
Toph sightless eyes widen. Then a smug smile spreads across her face. If she pulls this off, she's going to be the best Earthbender not just in the world, but in history.
And maybe she's not so useless in here after all.
May throws a set of knives. Toph focuses on the metal and throws out her hand in front of her. The knives reverse their course and whiz by Mai's head, embedding themselves into the metal wall behind her.
Ty Lee gasps. Toph grins. She turns and throws out her arms, willing the metal railing behind her to detach. It comes off agonizingly slowly - metal is even harder to manipulate than earth, but Toph is tougher. Then she throws the railing towards the girls. It wraps around their waists and the ends attach themselves into the far wall, trapping them.
"Thanks for the suggestion!" Toph says cheerfully.
Sokka and Suki stumble over while the Fire Nation girls struggle against the metal rod.
"Did you guys find Katara?" Toph asks.
"Yeah, but she disappeared. We think she went after Azula," Suki says.
"Then let's go!" Toph is already turning when Sokka reaches out and grabs her arm.
"Wait," he says. "We still have to stop the drill before it reaches the city."
"How are we going to do that?"
"I thought Katara could use her waterbending to cut the support beams from the inside. But now that you can bend metal…"
Toph understands immediately. She feels her way to the first of the support beams, places her hand on it, and then shoves her palm forward. The metal follows her motion, shooting forward and digging deep into the ground. The drill shudders but continues to keep moving. Toph moves on to the next beam and the next, turning the support beams into pegs that hold the drill tight into the earth.
Finally the drill shudders to a stop, unable to move farther. For good measure, she bursts a few pipes that Sokka says look important.
"Now let's go find Katara."
Katara ran out of the console room as fast as she could. She trusts Sokka to come up with a plan to stop the drill. Right now, she has just one person in mind: Azula.
She finds her way out of the drill, blasting any workers that get in her way with a whip of water. She emerges on the top of the drill, where she'll have a good view.
She sees Mai and Ty Lee facing off against Toph and a group of Kyoshi Warriors. They're holding their own, so runs farther down the drill. There's a thud and the entire drill shudders. Katara loses her balance but manages to stay on top of the drill. When she climbs back to her feet, she sees that the drill has breached the wall.
She also sees an unmistakable blast of blue fire. She narrows her eyes and runs towards it.
They don't notice her at first. An orange blur that has to be Aang is blasting air attacks at Azula, which she deftly deflects and returns with flames. He's barely holding his own.
Katara sends a whip of water at Azula's legs from behind. The princess, who hasn't seen her yet, falls to her knees. Aang catches a glimpse of Katara and a bright smile breaks across his face.
Azula takes advantage of his distraction and sends a wave of fire. He puts up wind to diffuse the flames, but the momentum from the attack slams him backwards into the wall. He falls to his face and his body begins to disappear as the drill pushes further into the wall.
Katara aggressively attacks Azula without relent. The princess is able to counter and return every attack.
Aang disappears under the wall.
Katara creates an ice block to surround Azula. Before Azula can melt it, Katara sprints to the wall and reaches her arm under, searching for Aang. She finds a piece of his tunic and pulls him out.
Azula bursts out and smiles triumphantly at Katara.
"The drill has breached the great wall of Ba Sing Se. You have no back up. And you're out of water." She takes a step forward. "You've lost, Avatar."
Suddenly the entire drill lurches to a stop. Azula, mid-step, loses balance and falls over the side. Katara drops to her knees and holds onto Aang with one hand and the top of the drill with the other. When the shaking stops, she climbs with Aang down the side and onto the ground.
Azula is lying on the earth, her body unmoving. Before Katara can check to see if she's even still alive, something punches through the metal from the inside of the drill. No, not something - someone.
Toph, Sokka, and Suki descend. As the approach, a group of Kyoshi Warriors and a few Earthbenders also run up. They form a circle around the fallen princess.
Azula raises her head and pushes herself up on her elbows. She examines her situation.
"What do we do with her?" one of the warriors asks.
"Nothing," Katara replies, her voice hard. "This one's mine."
Suki motions for her warriors to follow her. Sokka and Toph help Aang up and, after a nod from Katara, follow the others.
"You've lost," Katara says when the others are out of earshot.
Azula grins. "I've lost a battle. I haven't lost the war."
"I could end this war between us right now. Give me one reason I shouldn't." Katara holds her hand out, ready to deal a final blow.
"Because if anything happens to me, my dear brother dies. As it is, this little stunt is going to cost him dearly." Azula smiles sweetly.
Katara doesn't trust herself to not strike again. Every blow against Azula means punishment for Zuko. She can't risk hurting him. Not after he already risked his life to let her escape.
She simply turns on her heel and follows the others into the city of Ba Sing Se.