List of Ben 10: Alien Force episodes

Five years after the original Ben 10 series, a matured 15-year-old Ben Tennyson has removed the Omnitrix and lives a normal high school life. Visiting the Rust Bucket to see his Grandpa Max, Ben finds Max missing and encounters a mysterious alien which flees. Searching the RV, Ben finds a message left by Max claiming that he is investigating renewed alien activity on Earth, and that he has the Omnitrix. Ben confirms that the Omnitrix is still in his bedroom, and deduces that Max was hinting for Ben to put the Omnitrix back on. Ben consults his cousin Gwen, and the two are confronted by Magister Labrid, a Plumber who was aiding Max in his investigation before going missing. They agree to work together to find Max, and Ben puts the Omnitrix back on. The trio ambush an illegal trade between the Forever Knights and a group of the aliens who attacked Ben. During the battle, the Omnitrix recalibrates, giving Ben access to ten new alien forms. Ben is then attacked by his old nemesis Kevin Levin, the broker of the trade, whom he defeats. Kevin reluctantly agrees to help track down the Forever Knights who had fled, and the group travels to a Forever Knight castle, where they encounter a robotic dragon.

Ben, Gwen, Kevin, and Labrid defeat the robotic dragon and the Forever Knights, but in the process, Labrid is critically wounded. Telling Ben to keep investigating the alien conspiracy and to believe in himself, Labrid dies. Kevin, taking Labrid's Plumber's badge, agrees to continue helping Ben and Gwen. They trace the aliens to a huge underground facility housing a massive spaceship. Gwen concludes that Max was previously in the facility, but left. Ben decides that they need to finish the mission, and they proceed deeper into the facility, where they encounter an army of aliens led by an enormous, virtually invulnerable Highbreed. Ben duels the Highbreed until their spaceship launches, and the Highbreed orders the destruction of the surrounding area to hide their activities. Ben is able to sabotage the ship just before it vaporizes a hospital, and it crashes and explodes in the desert. Ben, Gwen, and Kevin then decide to continue working together to find out what happened to Max.

In search of Max's whereabouts, Ben, Gwen, and Kevin receive a Plumber's badge signal and enter the town of Grover's Mill, where local authorities are in pursuit of a Pyronite (the species of Ben's alien Heatblast) who is allegedly burning crop circles with his powers. Ben subdues the Pyronite, who reveals himself as Alan Albright, a half-human; his father was a Pyronite Plumber who gave him his badge to protect him from being arrested for interstellar trespassing. Ben helps Alan escape from the police, and they investigate the crop circles, which Gwen deduces are actually circuit boards spread across the whole valley. They discover that they were created by the aliens they previously fought (known as DNAliens), which activate the circuit boards and cause a huge machine to rise and alter the weather to lower temperatures. Ben and Alan destroy the weather machine, and the sheriff, realizing that Alan was not to blame for the attacks, recruits him to help search for more aliens in the region.

Kevin meets with an old friend named Argit, who claims he knows the location of a particular piece of alien technology that Kevin is looking for. Needing something to bargain with, Kevin steals Max's RV, the Rust Bucket II, but after seeing the huge amount of Plumber's tech and weapons inside it, Argit double-crosses Kevin and steals the RV for himself. The team follows Argit with a tracking device that Kevin hid on the Rust Bucket's undercarriage and stops Argit, but Kevin storms away to seek out Vulkanus, who is in possession of the alien device. Kevin tries to make a deal with Vulkanus, but the latter forces Kevin to absorb a rare alien crystal called Taedenite, planning to use him as a living gem mine. Ben and Gwen arrive and defeat Vulkanus, and the alien tech Kevin sought is revealed to be a message from Max, who instructs Ben to put together a team of Plumber's children to prepare for what is coming.

While Ben obsesses over fulfilling Max's request to find other Plumbers' kids, Gwen and Kevin come at odds over their mutual romantic feelings and Kevin's inaction in asking her out. The team encounters a weakened high school girl who nearly causes a car accident, which is averted by a superpowered boy named Michael Morningstar, who reveals himself as another Plumber's kid. Michael explains that something is causing girls at his school to become zombified monsters, and they agree to investigate together. They find and battle a group of zombie girls, who escape after Michael causes debris to fall between them. Gwen, infatuated with Michael and annoyed with Kevin, agrees to go on a date with him. Kevin, distrusting Michael, convinces Ben to check on her with him, where they find Michael absorbing her powers and turning her into a zombie. Michael is revealed to have been the one zombifying his classmates, stealing their life force for themselves. Gwen is able to defeat Michael, and the girls take their life force back from him, deforming him in the process. Kevin then destroys Michael's Plumber's badge, and Ben apologizes for being too quick to trust Michael.

Gwen convinces Ben to ask out his crush, Julie Yamamoto, on a date to a nearby amusement pier. Meanwhile, an alien spaceship crashes on Earth an releases a small, symbiotic alien which tracks down and attacks Ben. The alien kidnaps Julie and lures Ben to the crashed ship, which Ben finds is on a self-destruct countdown, with a Galvanic Mechamorph (the species of Ben's alien Upgrade) is trapped inside. Ben is able to rescue the Mechamorph, Baz-l, and prevents the ship from exploding. Baz-l explains that the symbiotic Mechamorph, nicknamed "Ship," was sent out in order to find a Plumber to rescue him, and mistook Ben for one due to the Omnitrix. Baz-l repairs the ship and leaves Earth, leaving Ship in Ben and Julie's care. Ben and Julie then walk home together, while Ship flees.

While Ben, Gwen, and Kevin are taking a break at Max's favorite fishing spot, they see a mysterious cloaked woman leave a flower at the base of a nearby tree. When the team tries to meet her, she disappears. They decide to stake out the area for a while until the woman shows up again. When she appears, Ben and Kevin confront her, and she is revealed to possess magical powers like Gwen's. However, when she sees Gwen use her powers, she is amazed. Gwen's magical powers are revealed to be inherited from the strange woman - Ben and Gwen's long-lost paternal grandmother, Verdona, a powerful energy being called an Anodite. Verdona attempts to convince Gwen to break out of her human body and embrace her full Anodite power, but Gwen refuses. Verdona reluctantly leaves, but promises to return to visit.

After the team faces off against a Techadon robot, Kevin grabs the severed hand of the robot and places it in his trunk. At night, Ben's childhood bullies J.T. and Cash Murray see Kevin's car outside a restaurant, and decide to wreck it. They push it down a hill, smashing the trunk open. They look inside and grab the Techadon gauntlet before the car's alarm attracts Kevin. Back at their hangout, Cash puts the gauntlet on, planning on using its built-in laser blaster to destroy Ben once and for all and repair their reputation but the full robot armor begins to reform around Cash's body. In doing so, the arm messes with Cash's mind, making him more hostile than usual. Ben fights Cash, and J.T. attempts to reason with him. After remembering his friendship with J.T., Cash's willpower overrides the robot arm, and it gradually reduces in size to the point where Cash can take it off. J.T. and Cash walk away with a newfound respect for Ben.

The Incursean Emperor Milleous's daughter Attea is kidnapped by the bounty hunter Sevenseven. Unless she is returned, Milleous plans to destroy the Earth. The Emperor's majordomo, Raff, seeks out Kevin to help on the search. Using Gwen's powers to track her, they teleport to a nearby dam to rescue her. When the dam is broken, Ben uses the untested Alien X to try to fix it. The task proves simple thanks to Alien X's reality-warping capabilities, but Attea is yet again captured by Sevenseven and Ben finds himself locked in endless debate with the alien's other two personalities, Bellicus and Serena, unable to move or transform back without their consent. Meanwhile, Kevin and Gwen struggle against the Incursean forces attacking them at all sides. Eventually, Ben is able to convince Serena to transform him into Swampfire, and makes quick work of the enemies. The team completes the job successfully, and Milleous informs them that Earth will be spared.

Paradox and Azmuth arrive on Earth to warn Ben of the approaching Highbreed fleet. Gwen and Kevin gather all the allies they've met to counter the threat. Meanwhile, Azmuth reveals to Ben that the Omnitrix contains the DNA of every sentient species in the galaxy, 1,000,910 in total, and unlocks the Master Control so Ben can effectively combat the Highbreed.

Ben begins by recapturing the lost aliens of Chromastone, Spidermonkey, and Goop before taking on Vilgax. However, despite trying to use Jetray, Big Chill, and Humongusaur he is unable to defeat Vilgax. Gwen and Kevin show up with Way Big, who Ben scans and attempts to transform into, but he becomes Chromastone instead. Vilgax smashes Chromastone and Psyphon picks up the Omnitrix, but then Ben reappears as Diamondhead and defeats Vilgax, saving the Earth.

Ben and the team travel to an alien planet to resolve a conflict between the inhabitants. Kevin finds that Argit has been selling the weapons from one side to the other after painting them a different color. A temporary truce occurs, but as Ben attempts to shake hands with the leaders of each side, one side takes offense and the war restarts. Then Ben kidnaps the leaders of each side to attempt to force another truce, but they escape after blinding Ben with a flashlight. Ben transforms into Way Big and accidentally breaks a statue of the planet's former leader. With both sides fighting and angry at Ben, he flees the planet.

Vilgax asks for Ben's help to take down Zs'Skayr who has taken over his planet.

The Omnitrix malfunctions, turning Ben into a ruthless and aggressive alien while on an intergalactic peace mission.

Vilgax and Albedo return and team up to kidnap Kevin and Gwen so that they can take down Ben.

Ben gives up the Omnitrix and overcomes seemingly impossible odds to defeat Vilgax, who has betrayed Albedo.