After the final school bell rings, Benjamin "Ben" Tennyson is ready to head out for a camping adventure with his Grandpa, Max Tennyson, and unfortunately, his rival cousin, Gwendolyn "Gwen" Tennyson. After the trip to the campsite and the unruly aspects of dinner, Ben wanders out into the forest and meets up with a meteor bearing an alien device: the Omnitrix. The Omnitrix jumps onto Ben's wrist, and he now becomes the wielder of ten different alien superheroes. After experimentation and trouble with robot droids and bounty hunters sent by the evil Vilgax, Ben, Gwen, and Grandpa Max begin their odd summer camping trip.

After Ben saves a child and parent in a burning building, Gwen and Grandpa Max come along with more bad news. In addition, there is a mad scientist roaming around Washington D.C. with a device that can change normal animals to mutant giants and also reanimate fossils of dinosaurs. The mad scientist, Dr. Animo, first encounters Ben, Gwen, and Grandpa Max at the M-Mart. After a grueling battle without the help of a hero, they chase down Animo to the Museum of Natural History. Gwen is then taken by a mutant bird, and Ben and Grandpa Max save her. Dr. Animo is then caught and restrained from receiving "the award he deserves," the Verities Award for science. Ben learns though an important lesson here: you don't always get what you might deserve.

While camping and swimming at a lakeside campsite, Ben encounters a sea-monster in the lake right after his joke on Gwen. Ben manages to escape, but Grandpa Max and Gwen don't believe Ben about his sighting. The next day, they go fishing and meet their fishing captain, Captain Shaw, who identifies the sea-monster as the Krakken. Throughout this adventure, Ben will meet untrustworthy people, hulking monsters, and he will also encounter his new found form: Ripjaws.

While shopping at an ice cream store in the desert, robbers attempt to steal an ATM. Ben and Grandpa Max capture the villains, but this is only the lesser of their problems. Then they drive to a retirement center to visit Aunt Vera for the weekend, but something very strange is happening here. Ben finds out that there is an evil species of alien shape-shifters taking over the retirement center for food. How will Ben save the day from robbers, strange aliens, and cheek-pinching aunts?

Ben will find himself facing his toughest challenge yet, when Vilgax sends out three alien bounty hunters: the crustacean-like Kraab, the mechanical assassin Sixsix, and the mysterious Tetrax (credited as Hoverboard) - to recover the Omnitrix. He soon learns that there's more than one way to defeat an enemy, and that things are not always what they seem.

After having a gross-out contest with the kid in the car next to him, Ben finds a propane truck tipped over, about to catch fire and cause a huge blast. With that taken care of, Ben and the gang are off to Sparksville, a town which brings new meaning to the words "tourist trap." After exploring all of Sparksville's many attractions, Ben and Gwen head over to see "it," Sparksville's main attraction. When they finally see "it," they are thoroughly disappointed and decide to pull a little prank. After Ben turns into Four Arms, he looses control and sends "it" flying and destroying the town. Ben says he will move it back, but the Omnitrix powers down and Ben accidentally unleashes Megawhatt, a small little alien with a big sense of humor. Now, Ben must stop this creature from destroying Sparksville.

In a hotel, Ben spots an area where they are demonstrating a new game. He decides to use Ghostfreak's power to sneak inside. He is caught, and Ben, Gwen, and Grandpa Max are kicked out for Ben's misbehavior. After Ben and Grandpa Max argue, Ben decides to wander out into the city on his own. There, he meets Kevin, a sociopath who has the power to absorb any form of energy. However, after Ben reveals the powers of what the Omnitrix can do, he soon realizes that Kevin may only be after the powers of the Omnitrix, for his own doing. Ben learns that there are sometimes where you can't have everything your way all the time.

Ben, as Four Arms, thwarts an armored car robbery by a very tough female named Joey. During the fight, some of Vilgax's new and improved drones attack and Grandpa Max is injured. When Four Arms takes Grandpa Max to the hospital, Joey starts messing with the drones and is converted into a powerful alien cyborg named Rojo. With these enhancements, Vilgax is able to communicate with her and convince her to work for him. When she attacks Ben at the hospital, she is thwarted once again, but Ben is left thinking that it's too dangerous for him to be around others and runs away. During their final confrontation, it turns out to be the advice of Grandpa Max that helps Ben defeat her. As Upgrade, he is able to stop her, but not before receiving a disturbing message from Vilgax.

After dealing with some criminals at a dock, the trio finds their way to a circus run by a clown named Zombozo, much to Ben's chagrin. There, Ben's fear of clowns comes out as they explore. They find a freak show along the way, featuring a trio of mutants with incredible abilities. During Zombozo's show, Ben is too afraid to laugh and ends up leaving. His departure turns out to be for the best, since Zombozo drains the happiness of others to sustain and empower himself. Meanwhile, Ben finds the freaks robbing the town. Ben is able to beat them, but can't bring himself to fight Zombozo. He has to find a way, though, since Zombozo kidnaps Gwen and leaves the entire town in a chuckling daze.

A magician named Hex, whose powers come from mystical charms, is defeated by Ben using XLR8 and a little help from Gwen and Grandpa Max. Afterwards, XLR8 takes all the credit for the victory and even gets one of Hex's charms as a souvenir. When Gwen complains that Ben is getting all the glory and recognition, as well as all the cool stuff, he gives her the charm he got from Hex. The charm gives Gwen perfect luck in everything she does, and she becomes her very own superhero, Lucky Girl. Unfortunately, her luck has a price, and Ben is the one who ends up paying for it.

When Ben isn't tall enough for a ride at a water park, he decides to change into one of his alien forms to get even with the ride attendant. Unfortunately for him, the Omnitrix malfunctions and he ends up as Grey Matter. To make the situation worse, he is captured by an alien fanatic name, who decides to trade him in for fame and glory. Now Ben, as Grey Matter, must find a way out of this predicament, after being given to a secret organization for testing.

When Councilwoman Liang attempts to redevelop the downtown area, the demolition efforts disturb a man named Clancy, who has the ability to control insects. Meanwhile, as Ben stops a criminal as Upgrade, he spots at a nearby ice cream truck. When he climbs inside to get a snack, he gets himself locked inside and ends up catching a cold. The virus binds with the alien DNA in the Omnitrix, causing side effects, and weakening his normally powerful forms. On top of that, Clancy kidnaps the councilwoman and threatens the entire city with a meltdown at the nuclear power plant. Now, Ben must save the day while dealing with the strange side effects in his alien forms.

With Vilgax's injuries fully-healed, he decides to retrieve the Omnitrix personally. Ben gets a message from Vilgax in a dream about his imminent arrival. This worries Grandpa Max greatly, and he insists that they get to Mount Rushmore as quickly as possible. Unable to pinpoint the exact location of the Omnitrix, Vilgax begins to lay waste to nearby Rapid City in order to flush its holder out. He succeeds, despite Grandpa Max's protests, and manages to capture Ben. Meanwhile, Grandpa Max makes it to Mount Rushmore with Gwen and shows her a huge facility beneath the mountain. There, he's stored a weapon that can defeat Vilgax. Unfortunately, the weapon fails, leading to a showdown between Vilgax and the combined Tennyson family.