British spy Finn McMissile infiltrates an oil rig owned by a nefarious group of lemon cars to save his comrade, Leland Turbo. While on the rig, he witnesses the lemons, seemingly commanded by the sinister weapons designer Professor Zündapp, loading a disguised electromagnetic pulse generator meant to look like a TV camera into a shipping crate. Following the shocking news of Turbo's demise, Finn's cover is blown. In a daring escape, he tricks the lemons into believing he has perished.
After clinching his fourth Piston Cup, Lightning McQueen returns to the quaint town of Radiator Springs to unwind with friends. However, things take a turn when Italian race car Francesco Bernoulli issues Lightning a challenge to join the prestigious World Grand Prix, a globe-spanning three-race event orchestrated by former oil mogul Sir Miles Axlerod, who aims to promote his new eco-friendly fuel, Allinol. Without delay, Lightning, alongside his loyal companion Mater, as well as Luigi, Guido, Fillmore, and Sarge, sets off for Tokyo, the site of the tournament's inaugural race.
During a promotional event in preparation for the World Grand Prix, Mater's antics — including an ill-fated encounter with wasabi — lead to an embarrassing situation on stage, drawing unwanted attention to Lightning. While cleaning up the mess, Mater unwittingly interrupts a brawl involving American spy Rod "Torque" Redline and the lemon henchmen, Grem and Acer. In a twist of fate, Redline attaches a tracking device to Mater, leading Finn and his partner, Holley Shiftwell, to mistakenly identify Mater as the spy they are searching for. In a grim turn of events, Redline is captured and murdered by Zündapp, who discloses that Allinol becomes volatile when struck by an EMP. He reports to his elusive superior about Redline's passing of intelligence. In response, Holley teams up with Mater to thwart Zündapp's nefarious scheme.
As the race progresses, three competitors fall victim to the camera's explosive capabilities. Lightning comes in second place behind Bernoulli, a result of a miscommunication with Mater, who was caught in a perilous situation with Zündapp's goons. In a thrilling twist, Finn abducts Mater, and they make their daring escape aboard Finn's jet, Siddeley. Their escapade takes them to Paris to gather intelligence from Finn's old acquaintance Tomber, and subsequently transports them to Porto Corsa, Italy, to participate in the second race. Here, Mater sneaks into a lemons' meeting just as the camera triggers another explosion, culminating in a chaotic multi-car collision and Lightning's victory amid rising controversy over Allinol’s safety. In a surprising move, Axlerod chooses to rescind the Allinol requirement for the final race in London. However, with a plan to eliminate Lightning during the race in action, Mater's anxiety leads to a slip that exposes his secret, resulting in him, Finn, and Holley being captured and restrained inside Big Bentley, where he admits his true identity is far removed from that of a spy.
As the climactic race unfolds, Lightning momentarily leads before he runs afoul of an inexplicably malfunctioning camera. Amidst this peril, the lemons reveal to Mater that they have implanted a bomb in Lightning's pit area as a contingency. Driven by urgency, Mater breaks free just in time. Meanwhile, Finn and Holley manage to escape but realize the bomb has been covertly affixed to Mater's air filter instead. As Mater races down the track, Lightning chases him, seeking to apologize for earlier miscommunications, while Finn corners Zündapp. The odds seem stacked against them as the lemons overpower Finn, Holley, and Mater, though they're unexpectedly rescued by the residents of Radiator Springs and Sarge's military comrades. In an exhilarating turn of events, Mater and Lightning find themselves at Buckingham Palace, where Mater unveils Axlerod as the shadowy mastermind behind the chaos, confirmed when Axlerod is forced to disarm the bomb. It's revealed that Axlerod orchestrated the World Grand Prix as a facade to disparage alternative fuels, undermining his competitors at the Tokyo race, ultimately pinning the blame on Mater. Following the arrests of Axlerod and the lemons by the Metropolitan Police, Mater receives a knighthood from the Queen, and he reconciles with Lightning.
Returning to Radiator Springs, Mater regales everyone with thrilling accounts of his adventures. Intriguingly, Fillmore shares that Sarge has swapped Allinol for a more eco-friendly organic fuel, shedding light on the previous camera failures. As a gesture of camaraderie, a "Radiator Springs Grand Prix" is organized, featuring contenders from the World Grand Prix. Finn and Holley invite Mater on another mission, but he opts to remain in Radiator Springs. As his gadgetry gets confiscated, he cleverly keeps the rockets and zooms off with Lightning just as Siddeley departs into the horizon.
