The movie starts with a snail named Theo (Ryan Reynolds) watching an Indy 500 race won by his idol, famed racer Guy Gagne (Bill Hader). Theo tries to emulate Gagne's mannerisms as he imagines himself winning. His brother Chet (Paul Giamatti) notices his enthusiasm and isn't too pleased.
Theo and Chet go to work in the garden they inhabit in a spot called The Plant. Chet picks out ripe tomatoes from the garden while Theo has to handle the overripe tomatoes. He takes one and imagines himself racing against it through the dirt. The other snails spot him and start laughing at him and causing him humiliation.
Chet keeps an eye out for the "Shell Crusher" - a kid on a tricycle who enjoys rolling over small creatures- and practices the tuck-and-roll technique to protect the snails from a predator. Theo retreats to the house to watch another Gagne race, listening to his rousing victory speeches while drinking an energy drink he endorses. As Gagne gets to the end, Theo spills his drink on the cable and short-circuits the TV. He tries to fix it but ends up pushing the TV off the desk.
The next morning, a very large and ripe tomato that the snails refer to as "Big Red" falls from its stem and rolls onto the outside of the grass where a man is mowing the lawn. Theo sees an opportunity to prove himself as he goes to fetch the tomato before the lawn mower gets it. Chet sees this and narrowly rescues Theo by shooting him with water from the garden hose. Big Red gets destroyed by the lawn mower, and the snails once again laugh at Theo.
Theo wanders off from the garden all the way to a highway. He sees what appears to be a shooting star and starts to make a wish, but it appears to be a plane. Then, Theo is pushed all the way off the bridge and onto a race car. He is caught in a street race and becomes excited to be in on the speed, until he is pushed into the supercharger of the car and has nitrous oxide poured all over him. He accidentally swallows some, causing a mutation in his DNA. He is shot out of the car's exhaust soon after.
Theo wakes up in a garage and is surprised when his eyes start glowing and his shell starts flashing. He makes it back to the garden and interrupts a meeting where Chet is speaking when his shell starts blasting music like a radio. Theo wanders out of the garden before the Shell Crusher. The kid tries to roll over Theo, but the snail finds that he sped himself to a safer position. The kid chases Theo all around until Theo uses his speed to manipulate the tricycle and scare the kid away. However, he rolls all the way past the fence and into the garden, destroying the community's food supply. This causes both Theo and Chet to be fired from the garden crew. As Chet angrily chastises Theo, he is snatched up by a crow. Theo rushes off to save his brother as other crows try to snatch him. Chet is dropped, but Theo manages to help him land safely. They find themselves in a lot full of garbage and other hazards for snails when they are caught by a guy named Tito (Michael Pena). He drives the snails to a shopping center called the Starlight Plaza.
Tito takes the snails to a room where others are gathering. They spot the snails and comment negatively on the two. Theo and Chet are placed alongside other snails who are equipped with custom shells. They are set to race on a little track spread out for them. Theo seizes the opportunity to show off his speed as he zooms all the way through the area, astounding both the humans and snails. With this, Theo reinvents himself as Turbo.
That night, the snail crew, led by Whiplash (Samuel L. Jackson), approaches Turbo. They are made up of Skidmark (Ben Schwartz), Burn (Maya Rudolph), Smoove Move (Snoop Dogg), and White Shadow (Michael Bell). Whiplash invites Turbo to join his crew, but Turbo comments on how slow they all are. Feeling challenged, the crew takes Turbo out and settle on a race to the top of the Starlight Plaza neon star sign. The snails help catapult themselves to the power lines and race each other. Turbo is amazed by this and speeds off to catch up with them. However, he is unable to get himself to the other side to reach the sign, and the other snails beat him. He remains impressed.
Throughout the shopping center are failing businesses. Among them is a mechanic shop run by Paz (Michelle Rodriguez), a nail salon run by Kim-Ly (Ken Jeong), and a sports shop run by Bobby (Richard Jenkins). Tito attempts to promote Turbo in order to attract customers to his taco shop, Dos Bros Tacos, which he runs with his brother Angelo (Luis Guzman). As Tito tries to think of the perfect way to show Turbo off, a truck with a giant poster of Guy Gagne promoting the Indy 500 pulls in, and Turbo zooms around the truck to show Tito was he's suggesting. Tito gets hyped and tries to convince his brother and the other shop owners to help fund this venture, but they want no part of it.
The snail crew spots a bus and causes them to break down at the Starlight Plaza, bursting their tires and bringing them before Turbo. Tito calls Angelo out to tell him they've got customers. While the people are entertained by Turbo, they buy tacos and receive services from the other shop owners, including Paz fixing the bus. With their collective profits, they decide to follow Tito and get Turbo into the Indy 500.
Tito, Paz, Kim-Ly, and Bobby travel with Turbo, Chet, and the snail crew to Indianapolis. When they arrive at the Indianapolis Motor Speedway, Tito attempts to be persuasive by wearing glasses to seem professional when he goes to register for Turbo's entry into the race. He is denied when it's learned that Turbo did not pass the rookie race. Tito is kicked out as Turbo spots Guy Gagne racing across the track, beating his own speed record. Turbo moves all the way up to be near his idol, pulling Tito with him. Turbo impresses the crowd as he speeds off on his own, traveling at 226 MPH, which is enough to allow him entry.
Footage of Turbo's racing hits the internet and he becomes a nationwide sensation. When the time comes for a decision to be made, the Indy CEO (Kurtwood Smith) denies Turbo entry. However, Gagne comes in and comes up with another rousing speech that convinces the crowd and the CEO to allow Turbo to compete. That evening, Turbo and Chet argue when Chet worries that his brother will be hurt and he would not be able to protect him. Turbo goes off into Gagne's room. The racer spots him near his trophies and starts talking to him like a fan. He suddenly becomes antagonistic when he tells Turbo that the world will be watching him compete because he is an underdog, but they will watch him (Gagne) win. This leaves the snail discouraged and let down by his now former idol.
The day of the race comes and Turbo is stuck behind due to the danger behind all the larger cars. He makes it to the pit stop where Tito and the shop owners try to get Turbo prepped up to continue, but the snail crew jumps in and gets him back out there after Whiplash tells him to race like a snail. With this in mind, Turbo easily maneuvers beneath the cars to get further ahead. Gagne becomes threatened and tries to get rid of Turbo. He slams him into a wall and causes a crack in his shell, slightly damaging his speed powers. He makes it back to the pit stop, and he is told that if he takes anymore damage, that'll be it for him. Paz tells Tito he has to end this, and he reluctantly tries to get Turbo out of the race, but the snail won't give up so easily, and he zooms back into the race.
In the final stretch, Turbo manages to get the lead over Gagne, to everyone's surprise. The racer makes another attempt to kill Turbo by slamming him against the wall, but he holds onto the rims. Gagne causes a major accident, and he manages to hit Turbo as the snail flies over his head, causing a larger break in his shell. A pile-up occurs, and Turbo is left ahead with his broken shell, his speed powers now completely gone. Unable to move any faster, Turbo dejectedly hides in his shell.
Chet, who has been watching the whole race in a cup, crawls all the way down to help his brother. Meanwhile, the snail crew is surrounded by crows, but Whiplash, ever fearless, tames the crows and meets with Chet. Together, they fly down to Turbo and give him the motivation and drive to continue. After some hesitation, Turbo starts moving on his own, drawing everybody back into the race. As Gagne comes to, he tries to drive his car over Turbo, but it backfires. Desperate, he gets out of the car and tries dragging it to the finish line, only to falter with every few steps. Turbo inches closer, and Gagne tries to step on him. Chet yells out "tuck and roll" to get Turbo to get into his shell and roll away from Gagne. With a slight push, Gagne inadvertently helps Turbo cross the finish line and allowing him to win the race. Everybody celebrates, even Angelo watching the race back home. Kim-Ly goes onto the track to beat Gagne up for trying to harm Turbo.
Sometime later, business in Starlight Plaza is booming. Tito buys Angelo a new stove with his money while Paz has hired people to help at her shop. Kim-Ly and Bobby also see new customers coming in. Even the snails from the garden have joined the crew. Chet has entered a relationship with Burn. All the snails have been equipped with custom shells to give them their own unique method of speed. Tito comes in and gives Turbo a new custom shell, and is surprised to see his shell fully healed. The snails are set up for another race, and Turbo suddenly begins glowing and revving up. His powers have returned, and he races off with his crew once again.
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