Dexter's Laboratory: Ego Trip, on Cartoon Network
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Caution: spoilers.
Based on the series Dexter's Laboratory. It starts with Mandark trying to steal the neurotomic protocore from Dexter. And failing. Once he's gone, DeeDee comes into the lab, and enters Dexter's time machine. After she disappears, a group of robots emerge from the time machine, to "destroy the one who saved the future." Dexter destroys them all fairly quickly, then uses the time machine himself, because he wants to see how cool he is, saving the future.
He emerges in his lab, which is now some family's rec room. When they learn he has no number (in lieu of a name), they call the police, from whom he escapes. But then a robot shows up, does a retinal scan, and calls him Number 12, and takes him to his work station. Which of course belongs to his future self. Who is a sniveling wimp. Number 12 has a menial job working for the company of which Mandark is president (and he likes to torture Number 12). Dexter has no idea how he could have turned into this wimp, but he discovers some brilliant plans for using the neurotomic protocore, hidden away. He decides to take Number 12 even further into the future, but they foolishly leave the protocore sitting out in the open, and Mandark steals it.
Further in the future, Dexter and Number 12 emerge in the lab, which is now a museum dedicated to Dexter, the omnipotent leader of Dextopia. The neurotomic protocore has been utilized to make the world a place of universal wisdom, peace, and harmony. And Mandark is now just a brain in a jar, who wants revenge against Dexter. Though the Dexter in this time is a senile old guy who can't remember how he saved the world. (Actually, he's just like Dexter was in one episode of the series where he tried to age himself so he could stay up late and watch TV, but ended up making himself too old. That's not important to the movie, but I thought I'd mention it because I always liked that they did that.) Anyway, now the three Dexters go into the past- sometime after Number 12's time, but before the omnipotent leader's time.
When they emerge from the time machine, Dexter's lab is in ruins. Everyone in the world is primitive and stupid, because the Overlord is hoarding all knowledge and technology for himself, forbidding anyone else from learning. They meet yet another Dexter, who is a totally buff hero. He relates exactly how things got this way. He wants to fight the Overlord, but his resources are limited. But the four of them go back to Dexter's lab, to build a robot to attack the Overlord. Of course, Mandark is the Overlord, and he's waiting for them- along with himselves from the other three eras we've seen, to fight the Dexters.
The battle reaches a stalemate, then DeeDee pops out of the time machine. I don't want to give away anything else that happens after that, but it's amusing. And the world is saved. And every part of the story comes full circle. Yep, it was a really awesomely cool and funny movie, one of the best things I think the series has ever done. I should also mention that it aired between seasons 2 and 3... and what little I've seen after Ego Trip was mostly inferior to the first two seasons.