A/N: Starting off right where the main story diverges from canon, which is where Zuko was supposed to show up at the South Pole.
Chapter 1: The Avatar Returns
At a small village in the South Pole, Sokka prepared himself for battle. His sister and that airbender had just set off a flare in the abandoned Fire Nation ship, and Sokka expected it to be seen by the Fire Navy. He had forced Aang to leave the village, and Katara had nearly left with him. Gran Gran convinced Katara to stay, and Sokka prepared the village for an attack.
But the attack never came. Not even a single flake of black snow from a passing Fire Nation ship. Sokka waited for hours, with no sign of any ships in the area. He continued to wait on the snow wall, adamant that there would be an attack. Sokka didn't want to admit that he was wrong, since that would mean he would have to apologize to Katara and Aang.
"Sokka come down from there!" Katara yelled. "You're going to miss your supper."
"And while I eat the Fire Nation could attack," Sokka yelled back.
"If there was going to be an attack it would have been here by now" Katara said. "Stop being stubborn and come down."
"Fine," Sokka said. He left his post, cleaned off his battle makeup, and quietly ate his supper. He could see Katara giving the 'I told you so' look the entire time. When Sokka finished eating he couldn't stand that look anymore. "Alright I'm sorry Katara."
"You should be apologizing to Aang," Katara said. "You ran him off over an accident, which by the way didn't cause any harm."
"Well he isn't here," Sokka said.
"He can't have gone far," Katara said. "We can find him and you can apologize."
Aang was taking a nap with Appa in an ice formation. Appa had been too tired to fly and Aang wasn't going to make him swim back to the Southern Air Temple. Both of them had slept for several hours, despite the sun still being in the sky. When Aang woke to his stomach growling, he considered going back to the village just for something to eat.
Who knew being frozen for a hundred years would make you this tired and hungry? Aang thought. He was about to walk back to the village when he saw Katara and Sokka wandering around the ice. "Hey!" Aang yelled to get their attention.
"There you are," Katara said as she walked up to Aang.
"Did something happen?" Aang asked.
"Nope, which is why we're here," Katara said. Then she grabbed Sokka and pushed him ahead of her. "Sokka has something to say."
Sokka looked at his boots and said "I'm sorry for kicking you out of our village like that. I was wrong and I realize that you're not a threat."
"Apology accepted," Aang said. "You wouldn't have something to eat by any chance?"
"I got some seal jerky," Sokka said. He took some out of his pocket and offered it to Aang.
"Anything not made of meat?" Aang asked.
"Meat is all we got," Sokka said. "You can't grow plants in ice."
Aang's stomach growled even louder. While he was a vegetarian and didn't want to eat meat, he realized that he really couldn't afford to be picky right now. He accepted the seal jerky and forced it down just to make his stomach happy. With that out of the way Aang asked "Hey do either of you want to come with me to the Southern Air Temple?"
"I'd love to," Katara said. "Afterward maybe you can take me to the North Pole to learn waterbending."
"Sure," Aang said. "Are you going to come too Sokka?"
"I don't think so," Sokka said. "Someone needs to stay here and protect the tribe. Now if you were the Avatar then it would be completely different."
"Yea, if I was the Avatar," Aang said with a weak laugh. "Well Katara do you want to go now?"
"Ok," Katara said. To Sokka she said "Take care of Gran Gran for me."
"Climb aboard," Aang said. He woke up Appa and hopped into the saddle with Katara. "Yip Yip" Aang said and Appa took off, surprising Katara and Sokka. When on their way to the Southern Air Temple Aang showed Katara a map of the world and pointed out all the places he wanted to visit. Appa flew straight for the Southern Air Temple, and arrived there in a few days.
When they arrived at the temple Aang showed Katara the sights. The airball field, the bison stables, the main courtyard, and then the statue of Monk Gyatso. After reminiscing about old times with Gyatso, Aang walked with Katara to the airbender sanctuary. Aang opened the door with airbending, and then went inside with Katara.
Inside were hundreds of statues, lined up in a spiral pattern. "Who are all these people?" Katara asked.
"I'm not sure," Aang said. Yet they seem so familiar he thought.
"Hey, they're lined up in a pattern," Katara observed. Pointing to four statues one at a time she said "Air, Water, Earth, and Fire."
"That's the Avatar Cycle," Aang said.
"Of course, they're Avatars," Katara said.
My past lives Aang thought. Then he walked to the last statue in the row, and zoned out while staring at it.
"Are you alright Aang?" Katara asked.
"Huh?" Aang said, startled.
"Who is that?" Katara asked.
"Avatar Roku, the last Avatar" Aang said.
"There's no writing," Katara observed. "How do you know his name?"
"I'm not sure," Aang said.
Katara was going to ask another question when they heard something enter the sanctuary. The two kids hid behind statues and waited for the trespasser to come into view. But it was just a lemur, which Aang wanted to claim as a pet. The lemur ran and Aang chased it through the temple, having fun until he followed the lemur into an old building.
Inside the tent were the remains of over a dozen firebenders. At the other end of the tent was a skeleton of an airbender, and Aang recognized the medallion it wore as Monk Gyatsos. Aang fell to his knees and cried over the loss of his mentor.
Katara followed Aang to that old building, and when she walked inside she found Aang in pain. Then she saw the firebender remains, and realized what Aang must be feeling. "I'm sorry Aang" Katara said as she placed a hand on Aang's shoulder to comfort him. Suddenly Aang's tattoos started to glow, and a maelstrom formed which destroyed the building and threw Katara away.
A sphere of wind lifted Aang into the air. The sheer power and the glowing tattoos were enough to tell Katara that Aang was the Avatar, and that this was the Avatar State. Katara knew she had to get Aang to calm down, and that she would have to talk to him to do it. "Aang, I know you're upset" she said. "I know how hard it is to lose the people you love. I went through the same thing when I lost my mom. Monk Gyatso and the other airbenders may be gone, but you still have a family. I am your family now."
Aang slowly descended and the maelstrom ended. The glow in Aang's tattoos and eyes faded as Katara walked up to him, and he collapsed from exhaustion into her arms. "I'm sorry" Aang said weakly.
"It's okay," Katara said. "It wasn't your fault."
Katara helped Aang walk down to where Appa was resting. Neither of them wanted to stay here any longer, and wanted to move on. Just before they left the lemur from earlier showed up, carrying a peach. Aang decided to keep the lemur as a pet and named it Momo.
As Appa flew away from the temple, Katara couldn't resist asking a question. "Why didn't you tell me you were the Avatar?"
"Because, I never wanted to be," Aang answered.
In the Fire Sage temple on Crescent Island, the eyes of Avatar Roku's statue began to glow. One of the Fire Sages witnessed the event, and sent word to the Fire Lord. From there news of the Avatar's return spread through the military, and a new standing order was issued. Neutralize the Avatar, take him alive if possible but if necessary, kill him.