In a suburban San Fernando Valley tomato garden in Los Angeles, Theo, self-named “Turbo”, is a garden snail who dreams of being the greatest racer in the world, just like his hero, five-time Indy 500 champion Guy Gagné. His obsession with speed often makes him an outcast in the slow and cautious snail community, and a constant embarrassment to his older brother, Chet. One night, Theo wanders onto a freeway to admire the traffic and wishes upon a star, later revealed to be an airplane, that he was fast. Suddenly, he is sucked into the supercharger of a Chevrolet Camaro drag racer, fusing his DNA with nitrous oxide. When he wakes up, he possesses superspeed and some characteristics of a real car. However, his first attempt to try out his new powers ends with him crashing a tricycle into the tomato garden in which many other snails work, resulting in him and Chet to get fired from the garden crew.
Chet angrily scolds Theo and ends up being snatched by a crow, but is pursued and rescued by Theo at a run-down strip mall called Starlight Plaza. There, they are captured by Tito, a taco truck driver, and are brought to a snail race held by him and his co-workers. Theo wins the race, astounding both humans and snails alike, earning the respect of the snails, led by Whiplash, and firmly establishing the name “Turbo” as his own. Tito dreams of reviving Starlight Plaza, in which his taco business is set up, with Turbo as an attraction, much to his brother and coworker Angelo’s annoyance. The snails eventually manage to divert and strand a tour bus, bringing impressive business. At this success, Turbo convinces Tito to enter him in the Indianapolis 500 as a competitor. The neighbors agree to put up the entrance fee and accompany them to Indianapolis.
Tito is initially refused entry into the race, but a chance meeting with Gagné gives Turbo a chance to show off his speed, astounding the audience and the racers by qualifying for the race via achieving a speed of 226 mph, becoming a sensation on social media and allowing the CEO of IndyCar to reluctantly let Turbo compete. The night before the race, after a heated argument with Chet, Turbo sneaks out to meet Gagné, who reveals his true colors by demoralizing him. In the race, the racetrack and the more experienced competitors leave him trailing in last place. At a pitstop, Whiplash and the crew give Turbo a vital pep talk, advising him to stop racing like a car. Turbo uses his small size to his advantage and rapidly gains ground, but Gagné resorts to cheating and knocks him against the wall, damaging his shell and weakening his superspeed.
In the final stretch with Turbo in the lead, Gagné, refusing to lose to him, inadvertently causes a pile-up involving most of the competitors, including Turbo, who wakes to find his shell demolished and his speed gone, so he falls into a deep despair. Alarmed at seeing his brother losing hope, Chet, with a change of heart, braves several dangers to meet with Whiplash’s crew. Seeing Chet and the crew arrive on crows to encourage him to continue, Turbo resumes the race. Gagné single-mindedly pursues him, dragging his wrecked car, but Turbo narrowly wins and Gagné is assaulted by one of the strip mall business owners for trying to harm him.
Starlight Plaza thrives from Turbo’s fame; all the businesses become spectacular successes and hold elaborate snail races. Whiplash’s crew are given special propulsion aids for their shells, while Chet is content with his new job as the track referee and paramedic. As for Turbo, he discovers that his shell has healed, and with it his superspeed has returned.
In a mid-credits scene, one of the snails from Turbo and Chet’s garden is finally able to tuck into his shell, only to realize that he is too big to come out.