Avatar: The Last Airbender (subtitle: The Legend of Aang in the UK) is a peculiar tv show. It aired originally (and, to this date, only) on the kid station Nickelodeon and was filled with childish humor, and yet still manages to have deep characterization and mind-numbingly intense action that will keep you on the edge of your seat. But I'm getting ahead of myself here. I'll start at the beginning.
Avatar takes place in a world separated into four nations: the Fire Nation, Earth Kingdom, Water Tribes, and Air Nomads. In each of these nations exist Benders, people who can manipulate their country's element (i.e, Benders from the Fire Nation can manipulate fire, and are called Firebenders) through spirituality and some awesome looking martial arts moves, all of which are based on real martial arts (for example, I'm fairly sure Waterbending is based on Tai Chi, although I could be wrong). This Bending drives the entire world; it's used for everything from warfare to delivering mail to propelling trains made of stone. However, Bending is a strict thing: not everyone who lives in, say, the Water Tribes can learn Waterbending, and no one can learn more than one kind of bending (although there are things like Sandbending and Swampbending; i.e, rarer but more specialized subsets of the larger groups) except for one person: The Avatar. Otherwise known as the Great Bridge between the Spirit World and the Mortal World, the Avatar is a person born once per generation who is able to master all four elements, and is generally called upon to mediate disputes.
Now that you have the premise, here's the plot, taken directly from the opening of each episode (because I'm not good at summing this up, I've tried):
"Long ago, the four nations lived together in harmony. Then, everything changed when the Fire Nation attacked. Only the Avatar, master of all four elements, could stop them, but when the world needed him most, he vanished. A hundred years passed and my brother and I discovered the new Avatar, an Airbender named Aang. And although his Airbending skills are great, he has a lot to learn before he's ready to save anyone. But I believe Aang can save the world." -Katara, Southern Water Tribe
That's basically the plot (or at least how it seems at the beginning, it gets more complicated) in a nutshell. Aang begins the story as a master Airbender, but has to learn the other three elements (in a specific order which also happens to be the order which also determines in which nation the next Avatar is born: Air, Water, Earth, Fire) in order to put an end to the war before Ba Sing Se, the capital of the Earth Kingdom and last real haven from the Fire Nation in the world, is breached. He travels with Katara (a novice Waterbender who sees this as her chance to learn Waterbending from a master, as there are no other Waterbenders at the South Pole) and Sokka (Katara's non-bending brother) of the Southern Water Tribe, and is hounded the entire way by the person who becomes by and large the most interesting character (for reasons that are, unfortunately, spoilerific), Prince Zuko of the Fire Nation, who was horridly burned and banished by his father and then told that the only way he can come back is to capture the Avatar.
The entire series is separated into 3 seasons, one for each element Aang has to learn, and it's truly interesting to watch how the characters change in that time; how Aang's character becomes less evasive right alongside his fighting style (Airbending is build on evading attacks rather than pure offense. Naturally, Earthbending is just the opposite), how Katara gains confidence as she becomes better at Waterbending and, perhaps most interestingly, how Sokka, the team's only non-bender copes with his relative lack of combat ability, striving to become the team's strategist to make up for it and, before that, making horrible jokes (even keeping in mind that they're mostly aimed at children) to draw attention away from his lack of ability. The fights in the series evolve the same way, starting as escapes from vastly superior and more numerous opponents, and eventually becoming even fights as the GAang (the fan nickname for the main group) grow in combat prowess (including Sokka, although it takes a while) and tactical ability (both due to Sokka's aforementioned drive to equal his teammates and the interesting perspective that mastering the elements gives Aang). Some of the fights by the end of the series are absolutely breathtaking displays, but it's a great testament to the series that this doesn't really make the early episodes suffer on a rewatch. The drama and tension is all still there even if you know what happens.
I said before that this show is shockingly mature given it's target audience, and that is true. It's the rare show that is aimed at kids without talking down to them and manages to give those kids' parents a reason to watch right alongside them. Sure, some of the jokes fall a bit flat on older audiences (although they did manage to sneak in some amazing Parental Bonuses [http://tvtropes.org/pmwiki/pmwiki.php/Main/ParentalBonus]. One minor character is a pastiche of Dwayne "The Rock" Johnson, no lie), and some kids won't understand all the character development, but there's one unifying feature: the combat. Sure, sometimes the series' combat suffers because it has to be aimed at a young audience (at least one death is made ambiguous when it could otherwise have been a very poignant scene, and all deadly weapons are guaranteed to never be used successfully on another person, at least directly), but for the most part, it all manages to be completely thrilling while bloodless. The three season finales, particularly, are well known for having spectacular fights, especially the second season's, which earned an Emmy for an episode called "City of walls and secrets", which is what I see as the beginning of the season 2 finale. It's frustrating to even mention this, as I can't even begin to explain what makes it so good without massive spoilers, but trust me on this.
As an unwritten rule, most shows on Nickelodeon have to try to teach morals, and Avatar is no different, at least for the first season. There's the standard morals about sexism and being careful, but they also throw in more complex morals, such as the ends not justifying the means, which would largely go over most kid's heads. It's also a testament to the show's phenomenal writing that these morals are never overemphasised, lost [http://tvtropes.org/pmwiki/pmwiki.php/Main/LostAesop], or broken [http://tvtropes.org/pmwiki/pmwiki.php/Main/BrokenAesop]. They effectively get their point across in a way that would probably really make kids consider it and wouldn't make older viewers roll their eyes. This is fortunate because, if you hadn't picked up on this from my review yet, it's rather clear that the show was made with several audiences in mind, which is a bad thing in less capable hands, but here the show's creators manage to pull of that balancing act. Even if they were unsuccessful here, though, from the second season onwards, every episode is needed for the plot and the morals stop appearing.
Recommendation:
Watch it as soon as you can (Here's [http://www.animefreak.tv/avatar/book-one-water] a good, legal site to watch it free), this show is excellent in a way I can barely do justice to. The writing is excellent, the jokes are good for kids, the action is good for everyone.

Avatar takes place in a world separated into four nations: the Fire Nation, Earth Kingdom, Water Tribes, and Air Nomads. In each of these nations exist Benders, people who can manipulate their country's element (i.e, Benders from the Fire Nation can manipulate fire, and are called Firebenders) through spirituality and some awesome looking martial arts moves, all of which are based on real martial arts (for example, I'm fairly sure Waterbending is based on Tai Chi, although I could be wrong). This Bending drives the entire world; it's used for everything from warfare to delivering mail to propelling trains made of stone. However, Bending is a strict thing: not everyone who lives in, say, the Water Tribes can learn Waterbending, and no one can learn more than one kind of bending (although there are things like Sandbending and Swampbending; i.e, rarer but more specialized subsets of the larger groups) except for one person: The Avatar. Otherwise known as the Great Bridge between the Spirit World and the Mortal World, the Avatar is a person born once per generation who is able to master all four elements, and is generally called upon to mediate disputes.
Now that you have the premise, here's the plot, taken directly from the opening of each episode (because I'm not good at summing this up, I've tried):
"Long ago, the four nations lived together in harmony. Then, everything changed when the Fire Nation attacked. Only the Avatar, master of all four elements, could stop them, but when the world needed him most, he vanished. A hundred years passed and my brother and I discovered the new Avatar, an Airbender named Aang. And although his Airbending skills are great, he has a lot to learn before he's ready to save anyone. But I believe Aang can save the world." -Katara, Southern Water Tribe
That's basically the plot (or at least how it seems at the beginning, it gets more complicated) in a nutshell. Aang begins the story as a master Airbender, but has to learn the other three elements (in a specific order which also happens to be the order which also determines in which nation the next Avatar is born: Air, Water, Earth, Fire) in order to put an end to the war before Ba Sing Se, the capital of the Earth Kingdom and last real haven from the Fire Nation in the world, is breached. He travels with Katara (a novice Waterbender who sees this as her chance to learn Waterbending from a master, as there are no other Waterbenders at the South Pole) and Sokka (Katara's non-bending brother) of the Southern Water Tribe, and is hounded the entire way by the person who becomes by and large the most interesting character (for reasons that are, unfortunately, spoilerific), Prince Zuko of the Fire Nation, who was horridly burned and banished by his father and then told that the only way he can come back is to capture the Avatar.

The entire series is separated into 3 seasons, one for each element Aang has to learn, and it's truly interesting to watch how the characters change in that time; how Aang's character becomes less evasive right alongside his fighting style (Airbending is build on evading attacks rather than pure offense. Naturally, Earthbending is just the opposite), how Katara gains confidence as she becomes better at Waterbending and, perhaps most interestingly, how Sokka, the team's only non-bender copes with his relative lack of combat ability, striving to become the team's strategist to make up for it and, before that, making horrible jokes (even keeping in mind that they're mostly aimed at children) to draw attention away from his lack of ability. The fights in the series evolve the same way, starting as escapes from vastly superior and more numerous opponents, and eventually becoming even fights as the GAang (the fan nickname for the main group) grow in combat prowess (including Sokka, although it takes a while) and tactical ability (both due to Sokka's aforementioned drive to equal his teammates and the interesting perspective that mastering the elements gives Aang). Some of the fights by the end of the series are absolutely breathtaking displays, but it's a great testament to the series that this doesn't really make the early episodes suffer on a rewatch. The drama and tension is all still there even if you know what happens.
I said before that this show is shockingly mature given it's target audience, and that is true. It's the rare show that is aimed at kids without talking down to them and manages to give those kids' parents a reason to watch right alongside them. Sure, some of the jokes fall a bit flat on older audiences (although they did manage to sneak in some amazing Parental Bonuses [http://tvtropes.org/pmwiki/pmwiki.php/Main/ParentalBonus]. One minor character is a pastiche of Dwayne "The Rock" Johnson, no lie), and some kids won't understand all the character development, but there's one unifying feature: the combat. Sure, sometimes the series' combat suffers because it has to be aimed at a young audience (at least one death is made ambiguous when it could otherwise have been a very poignant scene, and all deadly weapons are guaranteed to never be used successfully on another person, at least directly), but for the most part, it all manages to be completely thrilling while bloodless. The three season finales, particularly, are well known for having spectacular fights, especially the second season's, which earned an Emmy for an episode called "City of walls and secrets", which is what I see as the beginning of the season 2 finale. It's frustrating to even mention this, as I can't even begin to explain what makes it so good without massive spoilers, but trust me on this.
As an unwritten rule, most shows on Nickelodeon have to try to teach morals, and Avatar is no different, at least for the first season. There's the standard morals about sexism and being careful, but they also throw in more complex morals, such as the ends not justifying the means, which would largely go over most kid's heads. It's also a testament to the show's phenomenal writing that these morals are never overemphasised, lost [http://tvtropes.org/pmwiki/pmwiki.php/Main/LostAesop], or broken [http://tvtropes.org/pmwiki/pmwiki.php/Main/BrokenAesop]. They effectively get their point across in a way that would probably really make kids consider it and wouldn't make older viewers roll their eyes. This is fortunate because, if you hadn't picked up on this from my review yet, it's rather clear that the show was made with several audiences in mind, which is a bad thing in less capable hands, but here the show's creators manage to pull of that balancing act. Even if they were unsuccessful here, though, from the second season onwards, every episode is needed for the plot and the morals stop appearing.
Recommendation:
Watch it as soon as you can (Here's [http://www.animefreak.tv/avatar/book-one-water] a good, legal site to watch it free), this show is excellent in a way I can barely do justice to. The writing is excellent, the jokes are good for kids, the action is good for everyone.