Aristotle on Avatar: Soul's Construction

"The Plot, then, is the first principle, and, as it were, the soul of a tragedy."
               ~ Aristotle, Poetics, Book VI.

Therefore, as the plot is primary in importance, according to Aristotle, it shall be first in our analysis. To the plot then we first turn our gaze.
SPOILER! SPOILER! SPOILER! SPOILER!
The following analysis contains elements that may ruin surprises, and hinder enjoyment of the show if you have not already seen it and I highly recommend seeing it.

One point about a plot is that it must be made of a beginning, middle and end. Now that may sound self evident, and it somewhat is, but still so many storytellers fail to really follow this basic and necessary construction. I argue that Avatar, however, follows this principle. The beginning of the show is in the frozen lands of the northern water tribe where two young siblings hunt for food and inadvertently discover the long

lost Avatar who will bring peace to a world troubled by war. In later episodes, back story is given for many of the characters and for the land as a whole, but this is where the story begins from the view of the poet. What makes this a good beginning is not just that it is the first episode, but that it is the single event from which the rest of the story grows naturally. It is not just the first of many events, but the father of the events that follow.

A caveat that we must keep in mind is that there are many "plots" in this story. Many side stories and many anecdotes that perhaps deal specifically with one or several characters (e.g. Katara's seeking revenge on the man who captured her mother) but only deal indirectly with the plot.
What is the main plot, then? Well, if we look closely at the last battle between Avatar Ang and Firelord Ozai,

I think the main plot is revealed. The end of the last battle is not that the Avatar defeated the Firelord, but that he did so without killing him. The climax is when Ang defeats the Firelord with "soul-bending", something that none of the former Avatars had thought of. Something that Ang learned from the more "supernatural" world given him by the Lion-Turtle.

This same battle is the middle and climax of the main plot of the show. It is the one thing that the whole first part has been aiming at and the whole second part is the result of. It is, therefore, the middle. This makes a strong and clear middle that has both grown naturally from the beginning and leads naturally to the end. The end is also particularly strong because it shows such a restoration of peace and order, having tea in the city of Ba-sing-se which was the most contested of cities throughout the war with the fire nation.

The end, of course, is the end. This is, again, not because it is merely the temporal end, but because the peace and balance that is gained is the end to which the rest of the story has been purposely tending. It has been all along that the good would fight the evil for peace. Therefore, the beginning is the good coming (Ang in ice), the middle is the good fighting the evil (Ang and the Firelord), and the end is the good at peace (everyone having tea in Ba-sing-se). Thus, in construction of plot, Avatar: the Last Airbender series succeeds.