List of Ben 10 (2005 TV series) episodes

Ben Tennyson, a 10-year old begins his cross-country summer vacation with his cousin, Gwen Tennyson, and his grandpa, Max Tennyson. Ben finds a crashed pod at their first stop. Within it is a watch-like alien artifact which is called the Omnitrix. When he reaches to touch it, the watch jumps onto his wrist and refuses to come off, unwillingly merging with his DNA. Ben accidentally discovers that it can change his form into various alien creatures and quickly learns how to use it. Deciding that he could be a superhero who can actually help people, Ben uses the watch to destroy robots sent by Vilgax to retrieve it, thus beginning a summer of adventure for himself, Max and Gwen.

During a trip to Washington D.C., Ben confronts Dr. Animo, a disgruntled scientist whose Transmodulator can mutate living animals, and even resurrect dead ones, and eventually learns a lesson about what being a hero really means.

When Ben has an encounter with a lake monster called the Krakken while swimming in a lake. A part of the summer vacation involves a fishing trip, where they charter Captain Shaw's boat for the trip. However, Shaw has seen the Krakken times before, and is obsessed with catching it. Ben, with Gwen's help, soon finds out that the creature is upset because Jonah Melville a poacher in the disguise of an animal lover, is stealing the creature's eggs. After Ben retrieves the eggs, a close scene is shown with the Krakken cuddling her regained eggs in her nest.

Ben and Max visit a retirement center to visit their Aunt Vera, but find out that evil shapeshifting aliens called the Limax are kidnapping the 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. The Great One turns out to be impervious to Ben's original ten aliens, but 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, Wildvine.

As Ben and Gwen are planning Max’s birthday on the shore of Horse Thief Lake, a mysterious stranger appears and kidnaps her, so Ben follows her into a strange portal. When the dust settles, Ben realizes that he is twenty years in the future, where humans and aliens live peacefully. Ben then learns that Gwen's captor is her future self, who has become a master sorceress. Gwendolyn reveals that the future 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 a Navajo village owned by Max's old Plumber partner Wes Green in Chinle, Arizona, Ben falls in love with Wes' granddaughter Kai, while a wolf-like creature appears in a flash of purple lightning. Wes believes it to be a Yenaldooshi, or Navajo werewolf. It stole and ripped the receiver. Wildvine fights the creature, but it bites Wildvine's root just before Ben returns to normal. It nearly kills Ben, 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, and added its DNA to the Omnitrix by scratching it, thus gradually transforming 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 named 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; following a losing battle, they split up: Max and Gwen board the space shuttle that Viktor launches, and Ben as XLR8 follows Viktor, 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 Viktor 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.

During a trip in St Louis, Ben and Gwen meet a female alien named Xylene, who locates the Omnitrix and confronts Ben under the impression that he had stolen it. But when Max comes to defend them, he and Xylene recognize each other as old comrades and sweethearts. The two reminisce later that night, and 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 for safekeeping; Ben received the Omnitrix by mistake. But when one of Vilgax's remaining drones attacks and captures Max, Ben and Xylene reluctantly team up to save him, with Xylene unlocking Upchuck to defeat the drone.

Dr. Animo has escaped from prison with the help of a mutated seagull, plotting to get revenge on Ben, 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 the vastness of space to track down the Omnitrix's creator, Azmuth.