The movie opens in a vibrant world filled with anthropomorphic robots, where we meet Herb Copperbottom (Stanley Tucci), a dedicated dishwasher at Gunk's Greasy Spoon diner. He is ecstatic about becoming a father. After an arduous 12-hour ordeal, he and his wife, Lydia (Dianne Wiest), manage to create their baby, named Rodney (Ewan McGregor). Rodney grows up to be an innovative young inventor, driven by a desire to improve the world around him. He looks up to Bigweld (Mel Brooks), the brilliant inventor and founder of Bigweld Industries.
As Rodney matures, he creates a gadget called “Wonderbot” to assist his father with dishwashing duties. However, a mishap occurs when their boss, Mr. Gunk (Dan Hedaya), unexpectedly confronts them, leading to Wonderbot accidentally breaking dishes, ultimately resulting in Herb losing his job and falling into debt. Determined to make a difference and support his family, Rodney heads to Robot City with the hope of landing a job at Bigweld Industries.
Upon arriving, he meets Fender (Robin Williams), a quirky robot who makes a living by selling maps and taking souvenir photos. As Rodney eagerly reaches the gates of Bigweld Industries, he uncovers startling news: Phineas T. Ratchet (Greg Kinnear) has taken over the enterprise and is set to halt the production of spare parts for older models in favor of generating more profit through expensive upgrades. Those who resist this change are shipped to the ominous underground Chop Shop, run by Ratchet's fearsome mother, Madame Gasket (Jim Broadbent).
After being expelled from the company, Rodney finds refuge with Fender at Aunt Fanny's (Jennifer Coolidge) boarding house, mingling with other "Rusties"—older robots facing the imminent threat of the Chop Shop. Inspired by Bigweld's motto, "See a need, fill a need," Rodney starts to mend old robots himself. When Gasket learns of this and how it undermines Ratchet's plans, she orders Ratchet to eliminate Rodney and ensure Bigweld remains sidelined.
Rodney learns that his father has fallen ill due to his inability to obtain spare parts, spurring him to seek out Bigweld directly during the annual Bigweld Ball. Disguised to avoid detection, he and Fender are disappointed to learn that Bigweld cannot attend. Just as he's about to confront Ratchet, security intercepts him, but he's rescued by Cappy (Halle Berry), a smart executive at Bigweld Industries who secretly disapproves of Ratchet's greed.
Tragedy strikes when Fender is kidnapped and brought to the Chop Shop after escorting his love interest, Loretta Geargrinder (Natasha Lyonne), home. Though Fender endures a significant loss during his escape, he ultimately manages to free himself, albeit with a new pair of female legs.
Rodney and Cappy, determined to save the day, fly to Bigweld's home. In a twist of fate, Rodney inadvertently triggers a domino that leads to Bigweld's long-awaited appearance. Despite Rodney's heartfelt pleas to convince Bigweld to resume producing spare parts for the older models, Bigweld declines. However, after a motivational call from his father, Rodney rallies a group of Rusties to rise against Ratchet.
With a fleet of super-sweepers built by Gasket to eliminate the older robots, the stakes heighten. After some persuasion, Bigweld agrees to join Rodney and his friends in their rebellion against Ratchet's oppressive regime. The final showdown occurs at Bigweld Industries, resulting in Ratchet getting fired but not without a brutal fight.
As chaos unfolds, Rodney courageously rescues Bigweld and together they thwart Gasket's plan. In an intense climax, Wonderbot triumphantly vanquishes Gasket, while Ratchet’s defeat leaves him stuck alongside his father's outdated upgrades. Following the victory, Bigweld visits Rodney's parents in Rivet Town to share the news of their son's new role as his right-hand inventor and future successor.
Rodney surprises his father by gifting him a three-bell trumpet, which, through improvisation and collaboration, culminates in an impromptu celebration of music filled with jazz and funk. The film closes with a lively celebration as the audience dances to James Brown's "Get Up Offa That Thing". In a humorous finale, Wonderbot playfully tosses Timmy, the gatekeeper who denied Rodney access, into the air as he cries out, "I was on this list!"
