Ben 10, on Cartoon Network
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Caution: potential spoilers.
This 10-year-old named Ben Tennyson goes on a Summer road trip with his cousin Gwen (with whom he argues alot) and their Grandpa Max (a retired plumber). Meanwhile, an evil alien named Vilgax is trying to obtain a watchlike device called the Omnitrix, while I guess other aliens are trying to safeguard it, and before the device can be captured, they jettison it in a probe which crashes to Earth. Ben finds it, and it attaches itself to his wrist, and he can't get it off. He soon learns how to use the device, which gives him the ability to turn into any of ten different aliens with different powers, but the transformations only last 10 minutes (or so), and then it takes awhile for the Omnitrix to recharge or whatever, before he can transform again. Anyway, Ben has fun becoming a superhero and stuff (though he often misuses the Omnitrix for fun and pranks instead of heroics), and Gwen and Grandpa help out as much as they can (when not scolding him for misusing his powers).
I suppose I should also say that aside from Vilgax (who continues to show up from time to time), there will be a few other recurring villains. There's Dr. Animo, who turns animals into sort of weird, huge, monstrous "evolutions" of themselves, or whatever, and who wants to extract Ben's alien DNA from the Omnitrix. There's Kevin 11, who absorbed alien DNA from Ben and could transform into any of the original 10 aliens at will, though eventually that... went wrong. There's a sort of secret society called the Forever Knights, who collect alien technology, and want to obtain the Omnitrix. There's a magician (as in real magic) called Hex (who looks like a Voodoo witch doctor or something). He also has a niece called Charmcaster (who we like), who is of course also into magic. And there are probably a few other villains who will show up more than once, as well as characters who probably won't. Of course, not all guest characters (recurring or otherwise) will be enemies. Some may be allies, and others trickier to define. Like the government has a Special Alien Capture Team, led by Lt. Steel, who eventually learns Ben isn't the threat he first thought. And there's a group of alien super heroes called the Galactic Enforcers, who work with Ben on more than one occasion.
At one point, Gwen briefly got ahold of one of Hex's charms, and used it to temporarily become a superheroine called Lucky Girl. While this wouldn't last, it would hardly be her last brush with magic, and she'd eventually begin studying to become a magician herself, which of course makes her more helpful than ever (not that she wasn't always a great help, being smarter and more mature than her cousin). As for Grandpa Max, I think it's fairly clear from the beginning of the series that he knows more than he's saying about all the alien stuff going on, and by the end of the first season/start of the second, the truth will be fully revealed about Max Tennyson. (Spoiler: he was a member of a group calling themselves "Plumbers," who were actually something similar to Men In Black.)
Also in the second season, Ben will gain the ability to become some new aliens, as well as losing one (Ghostfreak, who becomes a new enemy). So... the show's title becomes a bit meaningless, as I see it. But that's a minor quibble. Throughout the series, Ben will continue to unlock new alien transformations from the Omnitrix, and in fact the first episode of season 3 showed the future, in which a grown-up Ben is known as Ben 10,000. (We also get to see a grown up Gwen, now a powerful full-fledged magician; who we like.) Also in season 3 we meet Xylene (who we like), an alien who used to work with Max in his Plumber days, and with whom he may have been romantically involved.
Hmmm, what else to say? In season 2, there was an alternate reality episode which reimagined the series starting with Gwen getting the Omnitrix instead of Ben. The series finale borrows the framework of that alternate reality, though I see the episode as regular continuity, and the true end of the series (as opposed to the TV movie Ben 10: Race Against Time). Ben returns to his hometown of Bellwood, we get to see his parents (Carl and Sandra). Ben has a final showdown with Vilgax. (No matter how many times he seems defeated for good, he keeps coming back, so I've just stopped questioning it.) Gwen's family moves to Bellwood in the end, and Gwen begins attending Ben's school. And... I could say more, but I don't want to spoil anything.
So that about wraps it up. Though I also wanted to say that the show's got a cool theme song.... And while the series started out slightly lame (but also slightly cool), I think generally the quality of the show has improved over time, somewhat. Certainly it keeps getting more complicated and interesting.... It still may have its flaws, but at least it's reasonably fun. Also, aside from the live-action "Race Against Time," there were a couple of animated specials. Plus, there's a sequel series called Ben 10: Alien Force, set 5 years after the original series. And later there'll be a third series, Ben 10: Ultimate Alien.