As the name implies, it's an alternate take on the series. To be more specific, Alternate History takes place in an Alternate Universe where Aang is awoken twenty years later than he is in canon. Zuko, Katara, and Sokka are in their thirties, while Aang is still physically twelve.
Zuko is still living much like he was as as teen—still on his fruitless quest for the Avatar. Or at least it was fruitless until he spotted a flying bison one night. Until recently, Sokka and Katara have resided in the withering South Pole despite many other Water Tribe members leaving the poles due to their desolate nature. After rescuing Aang, they've left home to go on a journey to save the world (with the possibility of finding a spouse on their trek being a bonus).
While the Gaang are travelling in the Earth Kingdom, they come across a group of Fire Nation prisoners. In hopes of finding their father, Sokka and Katara board Zhao's boat. There they meet a young woman named Yue who requests Aang's help in reviving the moon.
Alternate History provides examples of:
- Adaptation Relationship Overhaul: Katara being in her thirties, while Aang is only twelve, puts a wrench in the two having a romantic relationship. As a result, Katara is even more of a Team Mom towards Aang because she is old enough to literally be his mother.
- Alternate Universe Fic: Aang is awaken later than in canon. This causes major changes to the characters and the universe. For example, Katara is no longer Aang's love interest and, more importantly, the moon was captured years ago. Waterbending isn't possible without the moon.
- The Cameo: Song is the woman who gives Katara, Sokka, and Aang food on page 12.
- Children Are Innocent: Aang doesn't understand what a random soldier means when he says Zhao is especially fond of interrogating women.
- Death Glare: Katara delivers one to Zuko on the sixth page when he glances at her "family" for a few seconds too long.
- Dead Fic: The comic was never completed. It ended 48 pages in, right before the moon spirit is revived.
- Heir Club for Men: Azula already has three children but so far she's only given birth to daughters. She is having trouble producing a male heir. Of course, the actual show would later reveal that the gender of the Firelord doesn't really matter, but this was still in the early days.
- In Medias Res: Katara and Sokka have already met Aang by the start of the comic. They're travelling with him in the Earth Kingdom.
- Older Sidekick: Katara and Sokka are two decades older than Aang.
- Precision F-Strike: Katara's "Oh shit!" when Zuko recognizes her.
- Pint-Sized Kid: At twelve, Aang isn't even shoulder height to Yue.
- Relative Error: Inverted. Siblings Katara and Sokka are initially mistaken for Aang's parents.