Ben 10: Alien Force, on Cartoon Network
BCDB; Ben 10 Planet; Cartoon Network Wiki; IMDb; TV Tropes; Wikipedia
streaming sites: Amazon; Google Play; iTunes; YouTube
For more links see cartoon links.
Set five years after the original Ben 10 series. So of course, Ben and Gwen are now 15 years old (which means I can now say we like Gwen). Apparently the Tennysons haven't had adventures with aliens or the Omnitrix or anything for some time, though as of the end of the original series, it seems all that stuff was still going on. Anyway, it seems Max was recently investigating some renewed alien activity on Earth, and now he's disappeared. So Ben once again dons the Omnitrix, which now has a number of new aliens he can become. (He also seems to have become much more proficient in its use; he becomes the alien he intends, and the time limit seems gone, he switches between aliens quickly, etc.) Gwen, meanwhile, has apparently been studying Tae Kwon Do (or Karate or something), and more importantly, she's continued to develop her magical skills. Oh, and the two of them don't seem to fight with each other anymore; clearly, they've matured emotionally as well as physically. Though Ben also clearly still enjoys coming up with names for the new alien heroes he transforms into (it gets a bit annoying that he says the hero's name whenever he transforms).
Anyway, they try to find their grandpa, and get some unexpected help from Kevin Levin, aka "Kevin 11," who was involved in some alien weapons deal between the Forever Knights and an alien group called the Highbreed. (He was originally an enemy in the first series.) It's unknown how he escaped the Null Void, or what's happened to him, since he looks normal again, not all mutated as he was in the previous series; however, he can absorb the properties of any solid matter he touches, which certainly comes in handy. It should also be noted that he's obviously attracted to Gwen now, and the feeling is kind of mutual. So he's not an enemy anymore, though he and Ben still don't exactly get along. In any event, the three of them will not only try to find Max, but also try to complete his mission. I should also mention that Ben starts dating a girl named Julie Yamamoto (who we like), and she occasionally gets involved in all the alien stuff that's going on. Oh, and there's this alien animal kinda like a dog (but not really) which can transform into a spaceship. It belonged to an alien named Baz-L, but it ends up staying on Earth, where it becomes Julie's pet. She calls it Ship.
Well, there are tons of recurring characters, both good and bad. But the main characters are Ben, Gwen, and Kevin. The three of them will occasionally fight the Highbreed, or the Forever Knights, but there are other enemies to face, as well. And occasionally they meet descendants of Plumbers, who have powers of their own. We also learn that Max was involved with an alien (an Anodite) named Verdona, and they had two sons (Ben and Gwen's fathers). So Ben and Gwen are actually both part alien, though only Gwen has Anodite powers (which is where her "magic" comes from, though personally I think that doesn't completely explain it). We also meet a character named Professor Paradox, who was involved in a time travel experiment in the 1950s, and now he doesn't age and can travel through space and time at will. He pops up occasionally to advise Ben and his friends, when something of major significance is happening. At the end of season 1, Ben tries out a new alien hero, which he calls Alien X, whose power is virtually unlimited. But there's a major drawback, and he vows never to use that alien again.
Season 2 starts with a "magister prior" Plumber named Gilhil intending to arrest Ben, Gwen, and Kevin for impersonating Plumbers, though at the end of the episode, he instead drafts them, so they carry on as they have been. And um, I dunno what else to say except that by the end of the season, Ben and the others defeat the Highbreed. And I could swear I heard this was the series finale, but later they started a third season. Ben's pretty full of himself after saving the universe, but there are still alien threats to face. Max is back, and training some of the Plumber kids. Vilgax tries to take over the Earth, but fails. Ben gets in trouble with Azmuth, the alien who created the Omnitrix (first seen in the special Secret of the Omnitrix). Also this season, Kevin reverts to his mutated form from the previous series, which of course distresses him, but Gwen tries to find a way to make him human again.
At the end of the series, Vilgax teams up with Albedo (an alien first seen in season 2), to get the Omnitrix from Ben. Albedo had stolen an upgraded version of the Omnitrix, called the Ultamatrix, from Azmuth, but he needed the Omnitrix for a reason I won't get into. But ultimately, Ben ends up destroying the Omnitrix and obtaining the Ultamatrix for himself, which makes him more powerful than ever, as it allows his aliens to evolve into their ultimate forms. (In the original series, we got a glimpse of the future when Ben was grown up, but I don't remember if they said anything about an Ultamatrix in that episode, or if he was still using the Omnitrix.) Anyway, when the Omnitrix is destroyed, Kevin regains his human form and the abilities he had at the start of "Alien Force." Also, we get to see Vilgax's true form, which is kind of surprising... I had no idea the form we're used to wasn't his true form. And of course, he gets defeated yet again, but it's always possible he'll show up again, someday....
There is a live-action TV move, Alien Swarm, which must be set prior to season 3, assuming it can be considered canonical at all, which I doubt. And there will be a third Ben 10 series, Ben 10: Ultimate Alien, set shortly after the end of the second series....