Avatar: The Last Airbender, the Nickelodeon cartoon whose third season comes to an end tonight, is one part basic kids' cartoon and one part martial-arts extravaganza. The two-hour finale features an epic battle, lively humor, and philosophical questions that even adults will find themselves pondering.

So compelling is Avatar that it has inspired a Lord of the Rings-style live-action film trilogy from local fave M. Night Shyamalan, with the first movie set for release in 2010.

The series has been a hit pretty much from day one, winning multiple awards and garnering serious critical praise.

Avatar is the story of a world with four peoples: the Water Tribes, the Earth Kingdom, the Air Nomads, and the Fire Nation. Certain members of each group can control their particular element - water, earth, air or fire, "bending" it by using a set of martial arts moves and heaps of training.

All of which sounds hunky-dory except for one small problem: the Fire Nation is attacking everybody, and the Air Nomads have been wiped out.

To the rescue comes Aang, the Avatar, a 12-year-old boy who has the ability to control all four elements and thus restore balance to the world. The series premier found him popping out of an iceberg after 100 years spent frozen solid. Since then, audiences have followed his struggles to create peace and prosperity, despite his own preference for friends over fighting.

Tonight, viewers will get the large-scale battle they've been waiting for, as Aang takes on the cruel and callous Fire Lord. It is, to quote a popular song, "the ultimate showdown of ultimate destiny."

Avatar - televised on Saturday night, not Saturday morning - has been more than a kids' show right from the start, and it certainly doesn't start dumbing itself down now. Previous episodes have raised thoughtful questions about family, honor, spirituality and national duty, probing them with an adult level of understanding and a sense of humor that can appeal to all ages.

Now, Avatar looks into violence and morality, and Aang wrestles with the issue of justifiable homicide as deeply as any tenured philosophy professor.

Peer pressure pops up, too, as anyone and everyone tries to steer him away from what he knows to be right.

And, to the delight of anyone outside the 6-11 age range, all the sages in the "old people camp" get together to kick some serious butt, proving that just because you've outgrown PlayStation doesn't mean you're over the hill.

It isn't just the philosophical issues that are top-notch here, either. The animation far exceeds the standard Saturday-morning variety. The settings and characters are all depicted beautifully, obviously influenced by Japanese anime though the show is domestically produced. Many of its action sequences could even give computer-generated imagery a run for its money.

In the fashion of a true children's TV phenomenon, the show has produced action figures, video games, stuffed animals, even Lego sets. But Avatar has moved far beyond the generic "cartoon."