In a world populated by sentient robots, Rodney Copperbottom is an aspiring young inventor from Rivet Town who idolizes Bigweld, an entrepreneur and philanthropist whose company provides robots with products ranging from everyday appliances to spare parts. Following Bigweld's example to "see a need, fill a need", Rodney creates Wonderbot to assist his dishwasher father Herb. When Herb's supervisor confronts them, Wonderbot panics and wreaks havoc in the kitchen, leaving Herb in debt. To help Herb pay for the damages, Rodney decides to move to Robot City, hoping to present Wonderbot to Bigweld Industries in order to get a job there; despite objections from his mother Lydia, Herb encourages Rodney to pursue his dream, as he gave up his musical ambitions for his safer dishwashing job. Upon arrival at Robot City, Rodney is ejected from Bigweld Industries by his second-in-command Phineas T. Ratchet, who in Bigweld's absence has stopped producing spare parts in favor of expensive "Upgrades", thereby "outmoding" robots who are unable or refuse to pay for them. Ratchet's mother, Madame Gasket, runs the Chop Shop, a facility that collects scrap and spare parts with Sweeper trucks, and melts them to create Upgrades. Rodney meets ne'er-do-well Fender Pinwheeler under less-than-friendly circumstances; after fixing Fender's neck, he is taken in by Fender and his fellow outmodes, known collectively as the "Rusties": his sister Piper, Crank, Lugnut, Diesel, and their boarding mother Aunt Fanny. Word of Rodney's mechanical prowess spreads, and he is hailed as a local hero after he and the Rusties fix outmodes throughout the neighbourhood, although they are eventually unable to cope with the demand due to the spare part shortage. Rodney also receives news that Herb is in dire need of replacement parts. Hoping to enlist Bigweld's help, Rodney and Fender attend the Bigweld Ball (where he is reputed to make an appearance), only for Ratchet to announce that he will not attend. Enraged, Rodney publicly berates Ratchet, who orders his security team to eliminate him. Cappy, a Bigweld Industries executive opposed to Ratchet's plans, rescues Rodney and Fender. Fender is captured by a Sweeper and taken to the Chop Shop, where he discovers Gasket and Ratchet's plan to use a heavily-armed fleet of Super-Sweepers to destroy all outmodes throughout the city, and escapes. Meanwhile, Rodney and Cappy fly to Bigweld's mansion, where Rodney confronts Bigweld, imploring him to return to Bigweld Industries. Bigweld reveals that Ratchet's greed and business sense won over his idealism in the management of Bigweld Industries, and orders Rodney to leave. Rodney calls his parents, intending to return to Rivet Town, but Herb convinces him to stay. As the Rusties arrive to bid Rodney farewell, Fender reveals Gasket and Ratchet's plot; Rodney rallies Cappy and the Rusties to stop them. They are soon joined by Bigweld, who has regained his resolve. The group returns to Bigweld Industries where Bigweld fires Ratchet, but he is tricked and knocked unconscious. Rodney, Cappy and the Rusties (who have left Piper and Aunt Fanny behind for their safety) rescue Bigweld, but in a chase through the city, Ratchet lures them towards the Chop Shop, and Bigweld rolls into the processing area. After creating improvised weapons and "upgrades" from scrap parts, Rodney, Cappy and the Rusties confront Gasket before she can melt him down in a furnace, just as Piper and Aunt Fanny arrive with an army of outmodes. Cappy, the Rusties and the outmodes battle Gasket's henchbots while Wonderbot duels with Gasket, who is destroyed when she falls into the furnace; Rodney and Bigweld immobilize the Super-Sweepers and defeat Ratchet, whose Upgrades are destroyed in the process. Taking control of Bigweld Industries once again, Bigweld promises to make spare parts available to everyone. Later, Bigweld holds a public ceremony in Rivet Town, where he nominates Rodney as his new second-in-command and eventual successor. Rodney provides Herb with new replacement parts; as a final gift thanking him for believing in him, he gives him a trombone-like instrument to fulfill his dreams of musicianship. After a false start, Herb leads Rodney, Cappy, the Rusties, Bigweld and the townspeople in a rousing rendition of "Get Up Offa That Thing".
