In a world of robots, the Copperbottom family has a son, named Rodney. Growing up, Rodney becomes infatuated by famous robotics inventor Big Weld, and decides that he wants to be an inventor and work for Big Weld industries one day.
Rodney spends his youth and teenage years trying to perfect his own helper robot. He attempts to show his Dad that the robot can help him wash dishes, but helper ends up causing more damage than help.
Determined to make something of himself and pay off the damages the helper bot caused, Rodney decides to head to Robot City to fulfill his dream and work for Big Weld Industries.
Upon reaching Robot City, Rodney finds the gates to Big Weld Industries shut, with no new ideas coming in. The company, now led by a robot named Ratchet, has also scrapped the business model of supplying spare parts for older robot models, and going with pricier upgrades for robots, with a new slogan titled, "Why be you, when you can be new."
After being kicked out of Big Weld Industries, Rodney finds himself in a darker side of town, where a strange robot named Fender attempts to make off with his foot. Rodney chases after him, and finds a small collective of older robots scrounging for spare parts. In the ensuing chaos, Fender loses a part for his neck, and when the group goes to find a replacement part, finds this item has been discontinued. Even though Fender attempted to cannibalize him, Rodney helps Fender and manages to make a suitable neck replacement out of additional spare parts.
Fender's sister, Piper, explains that many robots in Robot City are in desperate need of spare parts that Big Weld Industries used to make. However, in recent months, Big Weld has disappeared; the streets are now patrolled by motorized vehicles, known as Sweepers, that take obsolete models to a dark side of town known as the Chop Shop, run by Madam Gasket (Ratchet's mother). Any collected models never come back.
Rodney bunks with the group and another robot named Aunt Fanny, but is surprised the next day when the local parts shop closes its doors, claiming that all spare parts orders from Big Weld Industries have officially dried up. All sorts of robot citizens are up in arms, but Rodney comes forward, and using what spare parts are left, helps those less fortunate.
Soon, the parts loss is felt at home, as Rodney receives a letter from his parents, saying that without spare parts, his father will not survive. Word soon reaches Rodney about an annual company ball held by Big Weld Industries. Rodney and Fender break into the festivities in hopes to talk to Big Weld and convince him to help, since Big Weld always attends this function.
However, Ratchet appears claiming that Big Weld couldn't attend. Rodney then comes forward, demanding to know where Big Weld is, and brings to the company's attention the plight of the average bot, of whom Rodney has spent several days trying to repair countless numbers of.
Ratchet orders Rodney be captured, but he, Fender, and a female employee named Cappy manage to escape the party. Fender parts ways with the group, while Rodney and Cappy head to Big Weld's mansion to investigate. Rodney is at first excited to meet his childhood hero, but soon grows disillusioned when Big Weld tells him to give up his fight to fix the outmoded robots. Big Weld claims that in the face of change for Big Weld Industries, Ratchet took over, turning the company into an entity that was more about profits than helping others.
Rodney then heads to the train station intent on going home, when the other robots and Fender appear. Fender, having just survived almost being destroyed in the Chop Shop, has overheard of the Sweepers being deployed in full force the next day by Madame Gasket and Ratchet to collect all the outmoded robots in Robot City.
Big Weld also shows up, and inspires the group not to give up; they all return to Big Weld Industries to confront Ratchet; Big Weld (inspired by the bots still believing in him) fires Ratchet, who weeps and begs for another chance; caught off-guard, Big Weld succumbs to a knockout punch from Ratchet, who then realizes he's become as crazy as his own mother!
Security arrives to remove Big Weld just as Rodney and Cappy turn up; between them they help Big Weld escape the building and end up being chased across Robot City, eventually ending up outside Madam Gasket's Chop Shop (although Big Weld goes shooting through the gates to the inside).
The group all tool up and enter the Chop Shop to rescue Big Weld from a certain melting down, but are faced with Madam Gasket, Ratchet and the army of new-fangled Sweepers. Piper turns up with all of the city's outmoded robots; working together, they attack Madam Gasket's outfit and rescue Big Weld. Madam Gasket gets melted down and Ratchet gets strung up in chains (next to his poor hen-pecked father) simultaneously losing his flashy upgrades! Big Weld, Piper, Fender and the others all celebrate a victory.
Rodney returns home (with Big Weld) and brings enough spare parts for his ailing parents; Rodney has been made Big Weld's right-hand bot, and eventual successor!