It begins innocently enough.

Aang and Katara are the ones who discover the shallow pool inside the Western Air Temple and call Toph, Zuko, and Sokka over to examine it. Its purpose had been, Aang informs them, to cleanse the body and mind of impurities during an important ceremony, such as the induction of a new monk trainee.

The Duke, Teo, and Haru are in the upper levels, exploring the vast meditation rooms. It is only Aang, Katara, Toph, Sokka, and Zuko who gather around the pool and blink down into its grimy depths.

Aang frowns into the water, looks around at the circle of weary, worried faces reflected in it, and comes to a decision.

With a flick of his wrist, he sends a cascade of the dirty water surging up into their faces.

By the time the four of them have blinked and coughed enough of the foul-smelling stuff away, Aang is poised above their heads atop a long, slender column of stone. His eyes are narrow, taunting.

"Aang!" Sokka shouts furiously, wiping at his face. "What was that for?" Zuko, for his part, shakes water off his hands and glares up at the Avatar.

"Twinkletoes," mutters Toph, still blinking furiously to ease the stinging water out of her eyes, "you are so dead." On the final word, she stomps her foot childishly and the entire column Aang is perched on explodes in a shower of dust and stone chips.

"Toph!" says Katara, shocked. "You shouldn't desecrate a temple!"

"Why not?" says Aang softly, from where he is clinging near the top of the wall, magnetized to the stone by earthbending. "The Fire Nation doesn't have any qualms about it."

Zuko flinches. Sokka's eyes widen. Katara stares at Aang, shocked.

Toph takes it as permission, and creates a wave of stone from the floor, sending it at Aang, who dodges neatly with airbending and lithely makes his way across the room.

"Stop it, Toph!" Katara admonishes her, and the blind girl subsides, grumbling. Katara turns to Aang, who is watching them with strangely glinting eyes.

"Come on, Katara," he says, and there is something in his voice that makes her pause, for just a second. "You're the waterbending master, right? Why don't you take a shot at me?"

"Take a—" Katara is stunned beyond belief. "Aang, why would I—?"

But her words are cut off when Toph earthbends a large slab of stone right out of the floor and hurls it at Aang with deadly accuracy.

In the resulting crash and shower of dust and stone fragments, Katara whirls and faces Toph. "What are you doing?" she shrieks. "You could have hurt him!" Even Sokka is now gaping at Toph in disbelief, and Zuko is looking warily from her to Katara.

That attack had been very serious in intent.

Toph doesn't answer. She is preoccupied with sensing Aang's position. Instead of dodging her attack this time, he barricades himself in a small stone fortress, which dissolves around him as the dust settles.

"Nice," she says quietly, and moves to attack again.

This time Aang blocks the barrage with a small cyclone, which spins the chunks of stone back towards Toph and the others. Reflex takes over; Toph shields herself with stone from the floor, Katara summons a great wave of water, Zuko lashes out with a flaming punch, and Sokka's boomerang knocks away a hefty-sized rock heading for his face.

They blink at Aang, stunned. All except Toph, who is concentrating on her next move, already painfully aware of where this is going.

Aang gazes back at them, delicately posed in one of his signature airbending formations.

"Aang?" says Sokka timidly.

He does not answer, only narrows his eyes a little.

And then Katara realizes: he is deadly serious too.

Aang is using his bending to attack them.

A sense of betrayal sweeps over her. She opens her mouth to ask why he would do this. But before she can utter a sound, Aang turns the water she is bending against her, catching her up in an airless cocoon. She accidentally inhales the murky liquid, chokes, and belatedly makes a sluggish motion that overrides his command; the water falls to the floor, and Katara falls to her knees, coughing up water.

"Aang!" Sokka rushes forward to stand in front of his sister, his eyes panicked and bewildered. "What are you doing?"

"My job," says Aang without inflection.

"Your job?" Sokka echoes, furious. "You just attacked my sister!" He glances down at Katara, who is now trying to call the water from her lungs with her bending. It is not a pretty sight. Sokka's eyes, now dark with anger, return to Aang. "Katara's hurt because of you!"

The words and the sentiment, painfully familiar to the three of them—Zuko and Toph wonder at the sudden chill in the air—echo in the wide room.

"Then protect her," suggests Aang, and sends a blast of that icy air straight at them.

Sokka's eyes widen; he is not braced for the attack.

Zuko leaps in front of them and shields them with a sheet of yellow-red flames. Toph stomps her foot again, and the stone underneath Aang crumbles.

He deftly leaps into the air and summons a ball of wind to hold his weight up; it churns dizzyingly beneath him. Katara, still trying to remove the last of the water from her lungs, recalls a time some months ago when that same ball of wind had been Aang's favorite toy, one over which he had not had much control and which had taken him careening into solid objects.

It is not a toy now, but a weapon, and he seems to have exquisite control over it.

Aang frowns at them, ranged almost side by side, unified against him.

"If you stand clustered together like that," he complains, "it's too easy to pick you off one by one."

To illustrate his point to his wide-eyed audience, he flips backward and simultaneously hurls the wind ball forward. It strikes Toph in the chest—without him standing on solid ground, she is unable to predict his attacks—and she goes down with a loud cry.

While Aang rebounds off the wall behind him, Zuko rushes forward and sends a bolt of lightning at him. For a moment, Aang's eyes widen in sensory memory; Azula's blue lightning dances in his mind. Then his eyes focus again, and he sends his own jet of white-hot fire at the bolt. The two energies collide in midair with a crack like a bomb exploding. Zuko, Sokka, and Katara are shoved backward. Aang hurtles through the air over the explosion and lands behind Zuko, in front of Sokka.

With a swift kick, he dislodges Sokka's boomerang from his hand. With a clap of his palms, a large boulder thrusts upward from the floor and takes Sokka with it up to the ceiling.

Zuko whirls around and shoots fire from his palms; Aang bends the water from the floor around Katara and extinguishes the flames.

Their eyes meet, gold boring into gray. They lower themselves into stances.

Katara struggles to her feet and watches as Aang and Zuko begin the Dancing Dragon. Although this time, they are not partners. They are opponents.

Fire seems to bloom from every available space around the two boys. Katara's eyes hurt from staring into the flames. She can hardly see their movements through the wall of burning red-orange-yellow. But she can hear plainly enough; Aang hisses when one of Zuko's shots skims his wrist; Zuko grunts when Aang's attack grazes his shoulder.

The ground beneath the two of them rumbles; the floor underneath Aang springs up like a trap to surround him and his fire; Aang is caged inside the rock with the flames. Toph has apparently recovered from the attack, and is very clearly furious.

Katara stumbles backward, away from the flame-wall and the cage of rock. Behind her, Sokka has scrambled down from the huge boulder, his skin scraped and bleeding in a few areas.

"What's gotten into him?" he pants. Katara cannot answer; she is just as lost.

"This is war," say Toph from behind them. They turn to stare at her. She has a huge bruise welling up under her jaw, but she looks as grimly determined as ever.

"This is Aang," Katara hears herself say.

"And Aang is at war," Toph retorts, her eyes narrow; her brow is soaked with sweat from the effort of holding the rocks together. Aang is probably launching a vicious assault from inside the cage. "And so are we. Zuko, surround the rocks with fire," she says.

Zuko, mouth creased in a thin, tight line, obeys; the cage of rock is soon engulfed in flames.

Katara starts forward. "No!" she cries. "That's—Aang is in there!"

"And he needs to get out," Toph says. "He started this battle; he'll need to finish it."

"This isn't a battle—" Katara begins, but stops herself.

This is exactly what it is, she realizes.

Aang is battling them.

The Avatar is battling his closest friends, his most powerful allies.

"Katara," says Toph quietly. "If he can't win against us, then how is he going to face the Fire Lord and the armies of the Fire Nation?"

"I—he—" Katara desperately searches for a reason to end this, to help Aang. "We'll be helping him."

"Not always," says Zuko, his eyes locked on the sight of the rocks and the fire. "There are going to be moments in the battle where he's on his own, and just as outnumbered as, if not moreso than, he is now."

"We're his friends," protests Sokka, but he too stops at the realization that if the Avatar cannot fight his own friends and win, he has no chance of defeating Ozai.

"We can't be his friends right now," says Toph. "We have to be his enemies if we want to teach him how to win this war. Think about it." Her voice is urgent. "We—you, Zuko, and I—are his bending instructors. He already knows that we take his powers seriously. Now he's going to have to take ours seriously too. We have to hurt him in order to help him."

Katara finds that she is crying, silent tears of grief and misery. What would it be like, she wonders, stricken, if the war had not been going on? They would probably have found this pool of dirty water, listened to Aang's offhand explanation, and then—what? Started a harmless water war? Bending would have come into play, but not intended to cause serious damage.

If the war had not been going on, she thinks, none of this would be necessary. Not friends fighting friends in order to learn how to survive, not children hiding in a deserted air temple, not a nation on the verge of conquering a world. Not Aang. Nothing would be as it is at this moment.

Katara closes her eyes and lets two last, bitter teardrops trail ghostly shadows of heartbreak down her cheeks.

When Aang explodes from the rocks and fire, encased in a pulsing shell of hot air, she opens her eyes and lifts her hands. A bullet of iced-over water catches Aang midchest and sends him crashing backwards into the wall.

While she hastily bends another powerful rope of water and stands at the ready, while Zuko imitates her with a long thread of fire, while Toph crouches warily and levitates large chunks of rock around her, while Sokka unsheathes his sword and falls into a fighting stance, while Aang shoots himself off the wall and simultaneously bends wind, water, earth, and fire at the four of them—Katara sees approval, apology, determination, and gratitude shining with the tears in his eyes.

--

Haru, Teo, and The Duke stare open-mouthed at the carnage of the room. What had once been a beautiful ceremonial hall is now in wreathed in ashes, rock fragments, and steaming pools.

The Avatar is lying on his back, breathing heavily. At various points around the hall, his four friends are also in similarly battle-worn states, although they are all on their feet, just barely managing to hold fighting stances.

"What—what happened?" croaks Teo. It cannot be as it appears—the Avatar's friends couldn't—wouldn't­—have ambushed him?

"Guys?" squeaks The Duke. "What's going on?"

"I lost," breathes the Avatar, eyes shut in defeat. At his admission, his friends all exchange glances from their stations around him, then slowly drop their guard and fall to the floor. Every single one of the five of them is sporting nicks, scrapes, bruises, dirt smears, and burns. Apparently, the Avatar has put up one hell of a fight.

"You—you lost?" Haru says disbelievingly. "So, what—this was training?" He glances around the wreckage. "You guys destroyed the ceremonial hall for training?"

"I wasn't training," the Avatar murmurs tiredly. "I was fighting."

"And you lost," Toph's voice points out weakly, but triumphantly. The Avatar lets out a single breath of laughter, then winces at some pain and falls silent.

The Duke is astonished. "You guys were really fighting?" he asks. "Like, for real really fighting?"

"Yes," Katara's voice drifts from her corner of the room. She sounds absolutely exhausted, and more than that, she sounds so achingly sad.

"You—hurt each other," Haru says, appalled. "You guys are friends, and you attacked the Avatar."

"He attacked us first," mumbles Sokka indignantly.

Teo, Haru and The Duke cannot think of a single intelligent thing to say.

"Tomorrow," says Zuko—the only one of the fighters who can speak with a clear voice, though he is breathing a little roughly, "we will fight again. Haru, Duke—"

They jolt at hearing their names. The Duke does not protest the missing article in his name.

"You will fight with us. You will use every resource available to you in order to defeat the Avatar."

They gape at the knowledge that they will be battling against their protector, their friend, their Avatar.

"Teo. You will observe the battle—the weaknesses and strengths of every person fighting. We will try not to harm you—it will be a test of our control."

Teo gulps at the responsibility—and at the possibility of injury.

"And Aang." Zuko pauses. "You will not lose next time." It is a dare, a command, a challenge.

The Avatar's eyes open and he stares up at the ruined, scorch-marked ceiling.

"No," he says. "I won't."