In Rivet Town, Rodney Copperbottom is a young inventor who dreams of making the world a better place. He idolizes Bigweld, a master inventor and owner of Bigweld Industries. During Rodney's adolescence, he invents a gadget called the Wonderbot, intended to help his father Herb clean dishes at a restaurant. When Herb's supervisor confronts them, Wonderbot breaks the dishes, causing Herb to be put in debt and Rodney to be dismissed. Rodney takes his invention to Robot City to see Bigweld and get a job as an inventor at Bigweld Industries, so that he can help Herb pay back his supervisor. Herb encourages him and confides that he has always regretted not pursuing his dream of becoming a musician. Rodney arrives in Robot City and meets outmoded robot Fender Pinwheeler. After a spectacular, but harrowing, ride on the crosstown express, Rodney arrives at the gate of Bigweld Industries.
However, he learns that Phineas T. Ratchet has taken over Bigweld Industries and is about to discontinue the manufacture of spare parts. Ratchet believes the company can make a bigger profit if it focuses on selling expensive upgrades; if the robots refuse, they are sent to the underground Chop Shop, where they are shredded and melted down by the owner of the Chop Shop, Madame Gasket, who is Ratchet's mother. Rodney is forced out and shares quarters with Fender at a low-rent boarding house owned by Aunt Fanny, occupied by a group of other outmoded robots threatened with the Chop Shop, called the "Rusties". When news gets out that spare parts have been discontinued by Bigweld Industries, Rodney begins fixing old robots on his own. Upon discovering this, Gasket orders Ratchet to stop Rodney and kill Bigweld.
Rodney later receives news that Herb needs replacement parts. Rodney decides to contact Bigweld directly to beg him to restart the manufacture of spare parts. Wonderbot reminds him that Bigweld will show up at the annual Bigweld Ball, which takes place that night. Rodney and Fender go to the ball in disguise, only to hear Ratchet announce that Bigweld was unable to attend. Rodney confronts Ratchet, but security robots stop him. Cappy, a robot executive who dislikes Ratchet's plan, saves him, and together with Fender and his new girlfriend Loretta Geargrinder, they escape from the ball.
After Fender walks Loretta home, he is taken to the Chop Shop by Gasket, but escapes. Meanwhile, Rodney and Cappy fly to Bigweld's home and meet him. Rodney asks Bigweld to return to his company and make spare parts available again, but Bigweld refuses, explaining that Ratchet forced him out of the company. Rodney calls his parents, intending to give up his dream and return to Rivet Town, but Herb encourages Rodney to continue pursuing it. Rodney rallies the Rusties into fighting back against Ratchet and Gasket. Fender reveals that, under Gasket's orders, Ratchet has built a fleet of super-sweepers with the intention of rounding up and destroying all of the older robots. Bigweld eventually comes with the group, having realized what Rodney meant to him.
At Bigweld Industries, Bigweld dismisses Ratchet, but Ratchet hits him over the head with a phone. This sends Bigweld insane and eventually leads into a chase. Bigweld is repaired just as they enter the Chop Shop, but is captured by Gasket. A battle ensues between the Rusties and Ratchet and Gasket's henchmen. During the chaos, Rodney rescues Bigweld, the choppers and sweepers are destroyed, Wonderbot kills Gasket by tossing her into the furnace, and Ratchet loses his upgrades. Retaking control of Bigweld Industries, Bigweld goes to Rivet Town to tell Rodney's parents that their son is now his right-hand inventor and eventual successor. Rodney makes his father's dream of being a musician come true by giving him a three-bell trumpet and the audience dances to James Brown's "Get Up Offa That Thing" as a victory celebration (sung by Diesel, having received a brand new voice box from Bigweld).
Who does Rodney Copperbottom idolize?
Bigweld