List of Ben 10 (2005 TV series) episodes

10-year-old Ben Tennyson begins summer vacation with his 10-year-old cousin Gwen and his grandfather Max. He wanders off and finds an alien pod that has crashed to earth, and within it a watch-like alien artifact. When he reaches to touch it, the watch jumps onto his wrist and refuses to come off. Ben accidentally discovers that it can change his form into various alien creatures and quickly learns how to use it, protecting people from robots sent by Vilgax to retrieve the watch. Deciding he can be a real-life superhero, Ben begins a summer full of extraterrestrial adventures for himself, Max and Gwen.

Ben overuses the Omnitrix and doesn't have any power left when Dr. Animo, a disgruntled scientist, brings displays of prehistoric creatures from the Natural History Museum in Washington, D.C. to life. Ben eventually defeats him, and learns that being a hero doesn't always come with rewards.

When Ben and Max go on a fishing trip with Captain Shaw, they encounter the Krakken that lives in the lake. With Gwen's help, they find out that the creature is upset because Jonah Melville, a poacher pretending to be an animal lover, is stealing its eggs.

Ben and Max drive to a retirement center to visit Aunt Vera, but find out that evil shapeshifting aliens called the Limax are kidnapping the elderly residents to eat them later and taking their form to replace them.

During a trip to Yellowstone National Park, Ben discovers the eleventh alien on the Omnitrix, Cannonbolt. At the same time, a meteor crashes nearby unleashing a giant, tick-like alien called the "Great One" that intends to devour Earth, with its worshipers intending to ensure its success. Ben easily defeats the worshippers, but the Great One turns out to be impervious to the original ten aliens. Ben quickly discovers that Cannonbolt is impervious from any attack when rolled into an armored ball and penetrates the Great One's armor in that form, killing the alien from the inside.

Distracted by Ben and Gwen quarreling, Max almost hits a young camper named Gilbert on the road. They take the boy back to his camp, Camp Opinicon, only to find no one there. As they investigate the premises, they find two more children, twins named Andy and Mandy, who are hiding from something that caused the camp to become deserted. Max is captured by tendrils while he investigates, prompting Ben and Gwen to split up. While Gwen leads the campers to safety from fungal mutants, Ben attempts two failed rescue missions, getting captured on the second. In trying to escape, Ben unlocks a new alien form, a plant-like alien named Wildvine.

As Ben and Gwen are planning Max’s birthday, a mysterious stranger appears out of nowhere and kidnaps her, so Ben follows her into a strange portal as XLR8. When the dust settles, Ben suddenly realizes he is twenty years later in the future, where humans and aliens live peacefully. Ben then learns that Gwen's captor is her own thirty-year-old future self, and a master sorceress to boot. Gwendolyn reveals that the current Ben, called Ben 10,000, is now a world-renowned hero with ten thousand different alien forms at his disposal. But, upon meeting his future self, Ben realizes that the future Ben is all business and just can't relax and have fun, not even for his own family.

During a visit to New Mexico, Ben falls in love with a Native American girl named Kai Green while a wolf-like creature appears in a flash of purple lightning. Kai's grandfather, Wes, an old friend of Max, believes it to be a Yenaldooshi, or Navajo werewolf. Wildvine fights the creature, but it bites Wildvine's root just before Ben returns to normal. It nearly kills Ben as a human, but he is spared when the wolf accidentally scratches the Omnitrix rather than Ben himself. Afterwards, Ben begins transforming into a werewolf himself, presumably an effect of the bite. However, the group discover that the werewolf is an alien. By scratching the Omnitrix, it added its own DNA to the Omnitrix, which was gradually shifting Ben into Blitzwolfer.

A shuttle launch at NASA is interrupted by a purple lightning storm, which attracts the attention of the Tennysons, who hope to discover the origin of the alien werewolf and mummy. While at NASA, Ben runs into an ill-tempered scientist by the name of Dr. Viktor. Surveillance tapes associate him with the purple lightning storm, and he is quickly shown to be in cahoots with the two creatures. The Tennysons encounter the creatures again, in addition to Doctor Viktor in his true form and, following a losing battle, they split up: Max and Gwen board the space shuttle that Viktor launches, and Ben as XLR8 follows Vicktor, who revives his master and commander, who to Ben's horror is revealed to be Ghostfreak.

Ben transforms into Grey Matter to escape from Ghostfreak and Viktor. He is able to discern Ghostfreak's plan from a computer terminal: using the corrodium to amplify the power source of the orbiting space station, in conjunction with the device the alien werewolf built in New Mexico, Ghostfreak is intent on covering the daylight side of the Earth in a corrodium shield. This will not only mutate Earth's population, but it will also block out the sun, giving Ghostfreak a world of eternal darkness to rule over. Ben uses Upgrade to create a makeshift rocket to get to the space station, which Vicktor clings to so that he might also get to the station, while Max and Gwen fight the mummy aboard the space center and attempt to thwart Ghostfreak's plan from there.

A female alien named Xylene locates the Omnitrix and attacks Ben, under the impression that he had stolen it but when Max comes to defend, the two recognize each other as old comrades and sweethearts. Xylene and Max reminisce later that night, which indicates they had been in quite a romantic relationship during Max's early Plumber days. Xylene reveals that she was the commander of the ship bringing the Omnitrix to Earth when Vilgax attacked it, and that she had intended to send it to Max; Ben received the Omnitrix by accident. One of Vilgax's remaining drones attacks and captures Max, so Ben and Xylene reluctantly team up to save him and Xylene unlocks Upchuck to defeat the drone.

Dr. Animo has just escaped from prison with the help of a mutated seagull, plotting to get revenge on Ben once again, who is at the beach with Gwen and Max. Ben is being impulsive as no one will play with him, until he is confronted by Animo's seagull. During the fight, Ben discovers a new alien on the Omnitrix, Ditto, who has the power to multiply at will. Ben manages to drive of the seagull and enjoys playing as his multiple Ditto copies. However, one of Ben's Ditto clones is captured by Animo himself, in hopes of exploiting his alien DNA for his own evil purposes.

During a battle with Dr. Animo, the Omnitrix is accidentally set to self-destruct. In order to stop it, Ben, Gwen, and Tetrax travel across space to track down the Omnitrix's creator, Azmuth.

Ben, Gwen, and Max, along with Cooper, use the Plumbers' Mount Rushmore installation to mount a final stand against the Forever King's forces. Max discovers that the Forever King was once a plumber named Driscoll who was kicked out after hoarding alien technology. Despite their best efforts, they are unable to keep Driscoll from obtaining the Sub-Energy. Driscoll uses the sub-energy to tremendously increase his armored suit and battles Ben, who is able to use Upchuck to digest the power source and destroy it safely.