Descartes wrote:
I never really took the time to watch that show because it looked stupid to me.
ZIM was supposed to be stupid. His people, the Irkans, survived by seizing other worlds, enslaving their people, and exploiting their resources. On ZIM's first mission, Operation Impending Doom, he activated his giant attack robot before the mission took off, devastating their base. He was exiled to Foodcourtia, the food-court world, but decided that when Operation Impending Doom II launched, he would stop being exiled and join in. Their leaders, the Almighty Tallest, decided to send him off to "conquer" a world off the edge of the map, which they had made up. Unfortunately for their plans, there turned out to be an actual world there - Earth.
ZIM has been trying to take over the Earth ever since, with the "aid" of his robot assistant, GIR (the units are usually designated SIR, for Standard Invasion Robot, but GIR was assembled out of junk parts found near the Tallest's podium). Given his height, he posed as a grade-school child. Most people are too unobservant (read: stupid) to notice that he has green skin and no ears, or are willing to believe his excuse that it's a skin condition. His archenemy is a classmate, a boy named Dib, who also has evidence regarding Bigfoot, the Loch Ness Monster, and Atlantis, so nobody listens to him.
The overarching plotline is I think best summed up by an exchange between Dib and his sister, Gaz, regarding ZIM:
"Can't you see it? He's trying to take over the world!!"
"But he's so
bad at it."
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.