You were ten when it all started. You wanted to go on a super awesome vacation with your Grandpa Max. Unfortunately for you, your cousin Gwen also had to come along. But you could stand that, if it meant that you got to do all sorts of cool stuff with your grandpa. You always looked up to him.
Naturally, that summer became one of the most interesting ones of your life. Starting with the discovery of the watch-like Omnitrix the first day you went camping. Of course, almost burning down a forest is not a great first start to using this new set of powers, but hey, it sure was fun. Once you figured out how to stop it, that is. Figuring out that you weren't going to be stuck as a fire monster, or rather, "alien" as Grandpa Max said, was another step in the right direction.
All this stuff was so cool for you. You had powers, stuff that made you cooler than other people. You could do things that others couldn't, even with the Omnitrix constantly timing out just when you needed it. So, of course you wanted to have fun. What ten-year-old wouldn't? Other than dweeb-Gwen, naturally. Of course you got the most buttheaded cousin, ever.
Eventually, you got sorta used to the fact that you could change species whenever you wanted to, if a bit annoyed that it wasn't the exact one you wanted. You had to deal with kicking some criminal(alien and human) butt from time to time, but you still got the chance to have your fun, if there was no one around. You always were in some kind of conflict on whether or not you wanted some secret identity. It didn't really matter, after all, most people didn't care after you transformed back into a kid.
It kinda bothered you, not getting any recognition. But it was something you had to deal with. Your first true fight was with Dr. Animo. He performed these...weird genetic experiments on animals. He could make them huge, he could bring them back from the dead, though they remained as zombies, which was both totally awesome and totally terrifying. He also seemed to have near perfect control over the animals, which was annoying. Especially when you wanted a one on one fight with him. It just wasn't fair. And it was beginning to turn out that, nothing would ever be fair again, now that you had the Omnitrix.
Your next big moment of realization came with Kevin. At first, he seemed cool. Another kid with powers! And he didn't have to deal with Grandpa Max telling him what to do all the time. Sure, you loved your grandpa with all your heart, but you wanted to use your powers for you, too! And that Sumo Slammers game was totally worth getting kicked out of a hotel for. But it turned out that Kevin wasn't such a cool guy. He just wanted to use his powers for himself, without caring about others, and he tried to steal yours. It was another important lesson to you, that you had to be careful with your trust. And yeah, maybe think before you used your powers for you again. You had to fight Kevin a few times after that, and he always scared you as a kid.
But your true enemy, the one you considered most dangerous, was Vilgax. He was cruel, and evil. A true warlord known throughout the galaxy. Coming for you, a ten year old who happened upon a watch. A space watch. With powers. That may or may not be the most advanced piece of technology in at least three galaxies, arguably five(the last two being heavily debated, of course.)
He is your greatest enemy, and the most vicious by far. He just wants to use the Omnitrix's power for himself, to make sure he can rule across galaxies. The Plumbers, an organization your grandpa had just recently told you about at the time, beat Vilgax back when Grandpa Max was a young man, Turns out, squid-y space lords like their revenge. And you had enough brute force to deal with it, as long as the Omnitrix didn't give you your wimpy aliens, like Gray Matter.
It made you realize that not everything was what it seemed. Your Grandpa wasn't just some (cool) old guy. He was part of some space police. The Omnitrix wasn't even meant for you. It was for him, and you just happened to be caught in the crossfire with some DNA that said "I'm at least partially Tennyson." Sometimes you wonder if you should take the watch off, but that's impossible. It's stuck on you now.
The rest of your summer began to blur together. You fought bad guys, your family helped, and you even got to meet some actual nice aliens. You learned things about the Plumbers, and about your grandpa. Things that kinda scared you. Not like you were gonna say anything, that was. You learned about the Null Void, a mostly prison-filled dimension where Plumbers sent those who they thought deserved it most. You even went there a couple times. It's where you left Kevin and Vilgax that one time, though Vilgax escaped. Vilgax always figured out more ways to outsmart you at the last second, and it drove you nuts.
You learned about the Forever Knights. They wanted what the Plumbers had, and eventually, most turned out to be alien haters, and they had a bunch of sub-factions or something(yawn!). Perfect for the boy who can turn into lots of different aliens! Not. Fighting them off from Plumber base(at Mt. Rushmore, by the way! You only destroyed that one twice. That...other time doesn't count.)
Eventually, your totally awesome roadtrip with Grandpa Max and Gwen ended. You knew that you would miss them, and even though Gwen was a total dweeb the whole time, you decided that she wasn't that bad. She helped you a lot of the time, and her ideas were also, well, smarter than yours.(But she'd never hear you say it, of course. You have a reputation.) You have to go back to school, and also probably think of new ways to say why your watch doesn't tell the time.
Even when you are 11, you still call yourself Ben 10. Get it? Ben 10, Ben Ten, Ben Tennyson? Hah. Anyways, you remain in Bellwood, mostly for school(blech!) purposes, though you do get to go on occasional weekend trips with your old team. And sometimes the villains are even nice enough to come to your town to do their various evils(thanks guys!)
You had to fight Malware a few times, some sort of Upgrade-like dude. You don't really remember it to well, and with the loss of your favorite alien, Feedback, you don't really care to. You fight Dr. Animo, Kevin, Vilgax, the regulars. Gwen can even participate, now that she's got use of her magic powers or whatever they are. It makes sense to you. This is what your "hero time" is. It's kinda what you live for.
But eventually, it comes to an end. You decide that you want the Omnitrix off. You're not really sure why. Maybe it's too much responsibility, maybe it's too much work fighting everybody. But, through a hard and lengthy process, you manage to remove the Omnitrix. It ends up in a shoebox in your closet, and for a while you can forget about all that alien stuff and go on being normal.
You still speak to Grandpa Max and Gwen, though. They respect your decision, and Grandpa reminds you that it's your choice if you ever want to put the watch on again. You become, if nobody minds you saying, the best
goalie that Bellwood has ever seen. You even earned a medal for MVP! Which happens to be the catalyst of your next set of adventures. When you went to show it to your Grandpa, you found him mysteriously missing, a recorded message all that was left. And then you got attacked. You'd never seen this kind of alien before, and it was odd. You managed to escape and grab the Omnitrix from home, though you decided to wait to put it on, talk to your cousin about what was happening.
That was when you met Fishface(Magister Fishface, that is.) He was your grandpa's partner on this case, and he thought you could help. You ended up with a stakeout on some illegal weapons deal, and still no Omnitrix on your arm. That is, until Magister jumped out and started several fires. And then the watch took its sweet time working, enough that the strange aliens and the Forever Knights could get away. But you did manage to get the broker, Kevin. Luckily for you, he hadn't been properly paid.
You end up destroying the weapons, and learning more about the new aliens in your watch. Magister ends up leaking his blood...water...stuff? All over the ground, and doesn't make it. You don't understand exactly what's going on, but you know you have to help out. After all, as Magister said, cracking this case could help you find your Grandpa. And hey, at least you make Kevin an ally now, but you're keeping a close eye on him. You still aren't sure that he's good for it.
You were the detectives of alien stuff from that point. You believed quite firmly that Grandpa Max was still out there, still alive. That he was leaving clues for all of you to find.