Theo, also known as Turbo, is a garden snail who wishes to be a racing champion, which has made him an outcast amongst the snail community and an embarassment to his brother, Chet. Turbo is demoralized when he is endangered trying to rescue a prize tomato and needs to be rescued by Chet. However, a freak accident causes Turbo to be sucked into the supercharger of a drag racer, fusing his DNA with nitrous oxide.
Turbo discovers that he now has incredible speed and accuracy. After he and Chet are fired from the garden crew by their foreman, Chet is snatched away by a crow. Turbo rescues him, but the two are captured by Tito Lopez, a truck driver, and are entered into a snail race. Turbo's speed astounds humans and snails alike, earning him the respect of a snail named Whiplash and his crew. This allows Tito to revive a strip mall with Turbo as an attraction. Turbo urges Tito to enter him into the Indianapolis 500 as a competitor. Though Tito's brother, Angelo, declines to support him, their neighbors agree to accompany them to Indiana.
At the Indianapolis, Turbo meets Guy Gagne, an Indy 500 racecar champion. Turbo's speed makes him a social media sensation, and Gagne pushes the CEO to qualify him for the race. However, the night before the race, Turbo is demoralized by Gagne and Chet. Undeterred, Turbo enters the race the next day, but in spite of his speed, he starts trailing in last place. At a pitshop, Turbo is given advice from Whiplash and his crew, who advise him to stop racing like a car. Turbo subsequently uses his small size to his advantage and gains first place. However. Gagne damages the snail's shell and causes a major pileup. With his shell punctured and his speed gone, Turbo begins to lose hope. However, he is reinvigorated by his brother and the crew, and continues towards the finish line. In desperation, Gagne attempts to kill Turbo, but instead knocks him over the finish line, allowing Turbo to win. Gagne is then beaten by Kim-Ly, an elderly manicurist.
After Turbo's victory, Starlight Plaza becomes a major attraction. Whiplash's crew gets propulsion aids to help with their shells in snail races, with Chet content as referee. Turbo later discovers that his shell has healed, and thusly, his superspeed has returned.