However, his life changed the day he went on summer vacation with his grandfather, Max and his cousin, Gwen. A short time after they left Bellwood, Ben went on a walk through the woods in which they were camping, and witnessed the crash of a strange capsule from space. This capsule happened to contain the Omnitrix, a powerful watch-like device, that firmly affixed itself onto Ben's left wrist.
At first scared, Ben soon discovered that, using the Omnitrix, he was now able to transform into ten super-powered alien creatures. Unable to remove the Omnitrix, with a stick he eventually learned to use the device (though with relative problems) in order to become a superhero. Though he occasionally arrested ordinary criminals, he was quickly led to also fight super-villains, starting with Dr. Animo, Kevin 11, and ultimately, Vilgax, a vicious and evil alien warlord who wanted to get the Omnitrix for himself.
Despite being very immature and occasionally using the Omnitrix for minor selfish purposes, Ben proved to be a valuable wearer for the Omnitrix, defeating Vilgax several times (though mostly due to the help from Gwen and Grandpa Max) and saving people several times.
In this series, Ben's favorite aliens are Four Arms, Heatblast, Wildmutt, XLR8, and Diamondhead, showing his tendency to favor brute force over well-thought plans. This was especially apparent when he was examining 16-year-old Ben's Ultimatrix; he called NRG and Big Chill "lame" when he saw them, called Brainstorm and Goop "weak", and said Humungousaur was "okay", probably due to his big size and stature.
In Ben 10: Secret of the Omnitrix, during a battle with Dr. Animo, Ben accidentally activated the Omnitrix's self-destruct and rushed off with Gwen and Tetrax to find the Omnitrix's creator. He was joined by Myaxx and later realized the countdown sped up when he transformed. He learned he could be a hero without the Omnitrix and Gwen nearly sacrificed herself to save Ben from some Floraunas. Ben then discovered that the creator of the Omnitrix was really a Galvan named Azmuth. He shut down the self-destruct and accessed Ben a new alien, a massive form named Way Big. He then defeated Vilgax and hurled him into space.
10-year-old Ben appeared in the Ultimate Alien episode The Forge of Creation. He looks different from the original 10-year-old version, as a result of the different drawing style. He now has a face that resembles the current Ben and appears slightly thinner as well. His cargo pants are shaded with a lighter shade of green and his shoes are different along with a skin color that resembles the present Ben's as well.