Invader Zim is a cartoon set in the distant future, and features the misadventures of the main character Zim. Zim is from planet Irk, & the Irken race appears to have conquered most of the galaxy.
Zim, though lackang any significant skill or fortitude, wants desperately to be assigned a planet to conquer as a part of Irken mission "Impending Doom II". Zim, having been a major hinderance to "Impending Doom I" is not allowed to participate in the former, unbeknownst to him.
The Irken race's society is comprised of a hierarchy based solely on height, with the tallest being the most respected, and the shortest being the least respected. Zim is... Short. The "Almighty Tallest" are the rulers of Irken society, as they are both the same height and the tallest living Irkens.
The Almighty Tallest send Zim (whom they know to be completely incompetent) to the very fringes of the galaxy to a planet they aren't even completely sure exists, which turns out to be earth. Zim, obliged to serve his master's request assumes the rank of invader and sets off for earth with a malfunctioning robot made up of spare parts and junk out of a trash can. The main goal of the tallest seems to be keeping Zim busy while they continue conquering the galaxy, because they know Zim will ruin everything if he's involved whatsoever. As a result Zim tries over and over to conquer earth unsuccessfully, even eventually learning to love some aspects of human culture, saving Earth on numerous occasions.
The often mentioned, and undeniable satirical element to this show shines brightly. The pet peeves of the intelligent are amplified to eleven and over acted to the utmost.
Since this world depicts Earth in a somewhat distant future, the people are all a bit dimwitted and simple despite having technology far advanced from our real life Earth. The show does feature some obviously advanced story-telling elements as read above, and can be a bit too creepy to look at for the overly sensitive. Basically if your kid won't watch Tim Burton films, they aren't the type to watch Invader Zim.
In the show, there seem to be only a handful of intelligent people, Dib and Gaz are siblings who both appear to be aware of Zim's real nature and intentions. Dib is a conspiracy theorist, as one of the only humans onto Zim, his main motivation is stopping Zim, although everyone thinks he's just crazy, and he doesn't know how incompetent Zim is. Gaz seems to be aware of Zim's intentions and just doesn't care. The only other two humans showed as anything resembling intelligent are the pair's father, Professor Membrane the world renowned scientist, and their "skool" teacher, Ms. Bitters.
Is it for everyone? No. Is it a bit dark? Yes. That being said, the overwhelming message of this show is to be aware, be accepting of others, think for yourself and ask questions. It's a show with a lot to say and creators who knew exactly how to say it. Kids from this generation may not get it, but when I was 8 or so this was my favorite show and it remains one of my favorite cartoons to date. Beware the borderline language. Your kid might take up calling people "FILTHY HUMANS!" as a joke, don't worry, they're not weird, you're weird.