After seeing the most famous robot in the world -- Mr. Bigweld (voice of MEL BROOKS) -- on TV, young Rodney Copperbottom (voice of EWAN MCGREGOR) has always dreamed of being an important inventor just like him. In need of money to pay for the dishes his latest invention broke, Rodney says goodbye to his parents, Herb (voice of STANLEY TUCCI) and Mrs. Copperbottom (voice of DIANNE WIEST), and rest of his hometown of Rivet Town, and sets out for the bright lights of Robot City.
He hopes to meet Bigweld and show him his invention, but Tim the Gate Guard (voice of PAUL GIAMATTI) informs Rodney that those days are long over. That's because Ratchet (voice of GREG KINNEAR) has taken over the corporation, much to the dismay of executives such as Cappy (voice of HALLE BERRY) who aren't happy that he's put greed over customer service as their new corporate mentality.
Accordingly, he's ordered that no more spare parts be made, and that if any out-dated robot can't afford the new upgrades, they'll literally be swept off the street and sent to the recycle bin. Little does anyone know that it's really Ratchet's evil mother, Madame Gasket (voice of JIM BROADBENT), who's running the show from her underground chop shop.
Meanwhile, Rodney is befriended by the loopy Fender (voice of ROBIN WILLIAMS), a scrounger who introduces him to his sister, Piper (voice of AMANDA BYNES), friend Crank (voice of DREW CAREY), and Aunt Fanny (voice of JENNIFER COOLIDGE) who possesses an extremely oversized derrière. Realizing that his new friends and other robots are in danger of being captured and recycled, Rodney sets out to repair them and thus becomes quite popular.
Word of his deeds soon makes its way back to Ratchet and Madame Gasket who orders her son to squash this industrial revolution. From that point on, Rodney must deal with them and their actions, all while trying to find out what they did to Bigweld and hoping to restore Robot City to its former, robot friendly grandeur.