Invader Zim is about an invader sent to Earth sent to decimate Earth. Well... not really, but let's play along with him.
Zim is an alien from the planet 'Irk'. On a planet where social status is determined by height, obsessed with snack foods, and in need of planets to conquer for single purposes; parking garage planets, or food court planets. Apparently, Zim thwarted the first attempt at galactic conquest, by basically destroying his own fucking planet, thinking it was a different world.
He was sent to do duties (I made a funny) on a food court planet, but left his own exile to try and help the second galactic conquest. Knowing he was too stupid to take on an actual assignment and too dangerous to be left on their own planet, the Tallest, Irk's political leaders, sent Zim to a small, under-the-radar planet called 'Earth', which is in no way important to galactic conquest.
Once on Earth, he meets a bunch of stupid, deformed creatures called 'humans', and is disgusted by their culture and behavior. This is where he meets Dib, a kid with an obsession with ghosts, aliens, trenchcoats, and apparently saw Bigfoot in his garage using a belt sander. Using his superpower of deductive reasoning, Dib points out some small, trivial facts, such as the fact that Zim is green, and lacks ears and a nose.
He attempts to capture, torture, and and destroy Zim and his plans to conquer Earth, but is thwarted by some small things such as a bird taking his camera and Zim shrinking himself in a microsubmarine and traveling inside Dib to destroy his brain. But overall, Zim is never very successful either, failing to conquer any substantial part of the planet.
Zim is accompanied by the robot Gir, whose CPU consists of: a piece of bubblegum, a paper clip, a bouncy ball, and a coin. This, following the laws of nature, causes him to act erratically, as if he had some devil spawn of Downs Syndrome and ADHD. As an example, he has a tendency to store Poop-Cola in his leg, tuna in his arms, and occasionally a sandwich or muffin in his head.
Although the show is considered by many to be a cult classic and a work of sheer genius, Nickelodeon has no sense of humor, and cancelled the show after a scant 27 episodes. Then, they went so far to restrict downloads of the show and Youtube videos. Creator Jhonen Vasquez's only comment was that he will eventually hug every Invader Zim fan for sticking with the show after its cancellation. As of yet, no hugs have been delivered. The bastard.