In a world of anthropomorphic robots, Herb Copperbottom (Stanley Tucci), a dishwasher robot at Gunk's Greasy Spoon diner, races through the streets of Rivet Town, elated that he is going to be a father. He and his wife, Lydia (Dianne Wiest), after 12 hours of "labor", finally manage to construct the baby. He is named Rodney (Ewan McGregor) and he becomes a young inventor who dreams of making the world a better place for everyone. Rodney idolizes Bigweld (Mel Brooks), a master inventor and owner of Bigweld Industries. During Rodney's adolescence, he invents a gadget, "Wonderbot" (Chris Wedge), intended to help his father clean the dishes at the restaurant. When Herb's grumpy supervisor, Mr. Gunk (Dan Hedaya), unexpectedly confronts them, Wonderbot panics and begins breaking dishes, resulting in Rodney being dismissed. Later, he decides to take his invention to Robot City to see Bigweld and get a job as an inventor so that he can help his family. He is encouraged by his father, who confides that he has always regretted not becoming a musician. Rodney arrives in Robot City and meets Fender (Robin Williams), a ramshackle robot scraping by through taking souvenir photos with a camera that has no film and selling maps to the stars' homes. While at the station Rodney meets our heroes, who have also come to Robot City. After a harrowing ride on the crosstown express with them, Rodney arrives at the gate of Bigweld Industries.
There he learns that Phineas T. Ratchet (Greg Kinnear) has taken over Bigweld Industries and is about to discontinue the manufacture of spare parts. Ratchet believes the company can make more profit if it stops making spare parts for older robots and focuses on selling more expensive upgrades. If any older robots protest, they are sent to the underground Chop Shop, where they are shredded and melted down by Ratchet's imposing mother, Madame Gasket (Jim Broadbent). (however when Rachet shares about the engines, Gasket insists that they should get them as well and us their metal to create more upgrades or better yet, a new kind of train system). Rodney and our heroes are forced out and finds a place to stay with Fender, at Aunt Fanny's (Jennifer Coolidge) boarding house, along with an assortment of other "Rusties", older robots threatened with the Chop Shop. When the news gets out that spare parts have been discontinued by Bigweld industries, Rodney remembers Bigweld's slogan, "See a need, fill a need", and begins fixing old robots on his own (with help from some of our heroes). Upon learning of this financial threat, Gasket orders Ratchet to stop Rodney and our heroes' work and kill Bigweld.
Rodney finds that Herb has fallen ill and cannot find replacement parts. Rodney decides to try to contact Bigweld directly, so he can beg him to make spare parts again. Wonderbot reminds him that the annual Bigweld Ball takes place that night. Rodney, our heroes, and Fender go to the ball in disguise, only to hear Ratchet announce that Bigweld was unable to attend. Rodney tries to confront Ratchet, but is stopped by security robots. They're saved by Cappy (Halle Berry), a beautiful robot-executive of the company who dislikes Ratchet's scheme and together with Fender and his new girlfriend, Loretta Geargrinder (Natasha Lyonne), they escape from the ball.
Fender walks Loretta home, but is captured by a Sweeper and taken to the Chop Shop. He struggles to escape, losing the lower half of his body in the process and eventually escapes by reluctantly attaching a new pair of female legs. Meanwhile, Rodney, our heroes, and Cappy fly to Bigweld's home. When Rodney inadvertently knocks over a domino, setting off a chain reaction of dominoes, Bigweld appears. Rodney tries to convince Bigweld to return to the helm of his company and once again make spare parts available, but he refuses, stating that Ratchet's money-making ideals have made him old-fashioned. Rodney calls his parents, intending to give up his ambition of becoming an inventor and return to Rivet Town, but his father again encourages Rodney to pursue his dream. Fender arrives and reveals that Ratchet has built a fleet of super-sweepers with the intention of rounding up and destroying all older robots, so Rodney rallies the Rusties to fight Ratchet. While Thomas, Twilight, And Spike contact for help. Bigweld eventually decides to come with the group, having realized what he meant to Rodney.
The group heads for Bigweld Industries, where Bigweld attempts to dismiss Ratchet. Ratchet damages Bigweld by hitting him on the head with a communications device, but escapes with Rodney's help. He is repaired just as they enter the Chop Shop, but is captured. Just then the Autobots appear ready to help our heroes battle Ratchet. A desperate battle ensues between Gasket's employees and Rodney's repaired robots. During the chaos, Bigweld is rescued by Rodney, the choppers and sweepers are destroyed, and Wonderbot kills Gasket by tossing her into the smelter, while Ratchet loses his upgrades and gets stuck on the ceiling with his father. 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 come true by giving him a three-bell trumpet, playing by sheer improvisation that prompts the others to help add other musical tunes; Fender finds the new music to be a mix of Jazz and Funk — "Junk". Bigweld gives Diesel a brand new voice box and with it, he sings James Brown's "Get Up Offa That Thing" as a victory celebration.
- Zecora, Stephen, Sharky, Mako, Willy, T.C., Hugs, Buzz, J.J., Socky, Skunky, 1206, The Miner Trains: (Steam Grindor, Steam Mech, Mucker, Steam Claw D., Steam Driller, Steam Sweeper, and Steam Exavator), Evan, Steamy, Puffy, Shai-Shay, Rattlesnake Jake, The Planet Trains: (Zip, Dazzlen, Dusten, Gustis, Rings, Uray, Tune, and Shiver), Dusty Crophopper, Bill and Ben guest star in this film.
- The engines are also targeted by Ratchet and Gasket so they can turn them into new upgrades
- This film will be dedicated to Robin Williams.