The film begins with Speed Racer (Emile Hirsch) sitting anxiously in a locker room before a race. As he taps his feet, we see a flashback that shows Speed as a boy, anxiously tapping his feet while taking a boring test in school. He tries to concentrate, but he finds the test extremely boring. Soon, he furiously starts to fill in answers after looking at the clock. Speed’s Mom (Susan Sarandon) is brought in to see his teacher, who’s concerned because his only interest is in car racing. Mom says that it’s because his father works on cars for a living. Speed’s teacher asks her who Rex is. Mom says that Rex is Speed’s brother, and asks her why she asked. It turns out that on the last test Speed took, he filled in all the bubbles so that it reads “GO REX GO”. Speed finishes his test, runs to the front of the class to drop it off, and runs outside to meet Rex (Scott Porter). Speed immediately runs around the car and hops in the passenger seat. On the way home, Speed begs for Rex to take him to the racing track with him. Rex eventually agrees, and they drive on the track as Speed sits on Rex’s lap. Rex teaches Speed some moves, and tells him that a car is a living, breathing thing. As they jump a ramp, we see the grown-up Speed racing on the track. Various announcers rapidly talk about Speed as the race goes on. One announcer makes mention that he hopes Speed is more careful than his brother was. Sparky (Kick Gurry), who’s watching the race from the sidelines, warns Speed of a racer heading straight for him. The driver tries to ram Speed, but he launches his car into the air, causing him to jump above the driver. The driver loses control and crashes.
As Speed continues to navigate the course, we see another flashback of Rex racing on the track. Speed watches with Mom and Pops (John Goodman). Speed maintains that Rex is going to set the track record, and he’s the best driver in the world. We also see that the other drivers are trying to take Rex out, but he’s too fast. Back to grown-up Speed’s race, the announcers are going crazy for him because it looks like he’s going to beat Rex’s track record. Spritle (Paulie Litt), Speed’s younger brother, tells Pops that he thinks he’s going to beat the record, but Pops tell him to be quite while he watches. Spritle doesn’t think he can watch anymore, and so he hands his binoculars to his chimp. Trixie (Christina Ricci) also watches from the stands, and we see another flashback. While young Speed races around in a goat-cart at a party, a snobbish girl insults Speed and his whole family. Young Trixie won’t stand for it, and so she punches the other girl in the face. This catches Speed’s eye, and he becomes so infatuated with her that he forgets that he’s driving. He crashes through a rose garden and Trixie rushes over to help him. Dazed, Speed asks Trixie who she is. She tells him that she’s in his class, and he asks if she wants to see his car collection. Speed drives Trixie to his house and introduces her to Pops and Rex. Speed says that a fan gave him a package to give to Pops. When he describes what kind of car he was driving, Rex grabs the package and puts his ear to it. He quickly puts it in Speed’s goat-cart, uses a mop to put it in reverse, and the package explodes in their driveway. Another flashback shows Rex packing up his things at night. Speed asks if he can come along, but Rex says no. He doesn’t know when he’s coming back, and so he gives Speed the keys to the Mach 5. Rex also says that one day people will say things about him, but he hopes that Speed won’t believe them. Speed promises that he won’t.
As Rex leaves the house, Pops confronts him about quitting and selling out. He tells him that if he walks out the door, he better not ever come back. Rex leaves. Some time after he leaves, the announcers spin many stories about Rex being a rat and being a dirty driver. Speed gets in a schoolyard fight with a kid who insulted Rex, but his teacher breaks it up. One day, the Racer family watches the news to see that Rex has been involved in a deadly car accident. Back to the grown-up Speed, he dashes for the finish line while a hologram shows Rex’s car driving next to him (from when he set the record). Speed purposefully lets his foot off of the accelerator so that Rex will still have the record. Racer X (Matthew Fox) watches the race on a TV, and he says that Speed will become the best racer if they don’t destroy him first. Next, we see Spritle playing with his chimp while watching cartoons. Mom catches him and makes him stop. A sponsor calls to speak with Speed, but Spritle hangs up the phone. He says that Speed has to race for Pops, but Mom tells him that Speed will have to decide on his own. While the family has breakfast, Speed wakes up and joins them. Sparky and Trixie are also present. Suddenly, the whole house begins to shake. Spritle thinks it’s an earthquake and takes cover. After it stops, they hear someone at the front door. Pops opens the door to see Mr. Royalton (Roger Allam) standing outside with his plane parked in their driveway. He allows Royalton to enter, and he immediately sweetens them up by giving Mom some flowers and Pops some cigars.
Mom has Royalton stay for breakfast and gives him some pancakes. After complimenting her for the food, he gets right down to business with Speed. He wants to sponsor Speed, but Pops remains adamant that they’ve always been independent. However, Royalton convinces everyone to accompany him to Royalton Towers to see his enterprise. Everyone is flown out to the city and they look at all the sights below. They arrive at Royalton Towers, and are given a tour of his building, where basically everything is much better than what the Racer family can do. They also see that every driver has to undergo rigorous, ridiculous exercises to stay in shape and be as confident as their vehicles. The tour is led to a fancy ballroom, where they meet Cannonball Taylor (Ralph Herforth), another famous racer. He races for Royalton and tells Speed that he’s come to the right place if he wants to win. Each racer gets a penthouse and several servants. Everyone is overwhelmed by the experience, but Pops says that he feels more intimidated than impressed. If Speed wants to be sponsored by Royalton though, he won’t argue. Speed walks in wearing a tailored suit and Royalton asks him what he thinks. Before Speed can truthfully answer, Royalton senses that he might say no, and so he tells him to think about it.
Somewhere around here, Taejo (Rain) gets beaten up by some men in an office (which is inside a big rig truck). They say that he hasn’t made good on their deal, and so they threaten to feed him to some carnivorous fish in a nearby tank. Just as they are about to do it, a car drives up behind them. They realize that it’s Racer X, and so everyone grabs weapons and tries to kill him. Racer X is able to outmaneuver them and fires guns into the truck. One of the bullets hits the fish tank, and one of the henchmen is forced to plug it up with his finger. The villains give up as the fish eat the henchman’s finger. They throw Taejo out of the truck and speed off. Racer X gives Taejo a lift and tells him to become smarter. After a while, he pulls over and forces Taejo to get out of his car. He says that if he survives the next race, he should call an inspector. Elsewhere, Speed and Trixie sit in the Mach 5 and contemplate Royalton’s offer. Shortly afterward they flirt with each other, and Speed promises that after he wins some big race, he’ll scoop her up in his arms and kiss her in front of millions of people. Just as they are about to kiss, they hear Spritle in the trunk with his chimp. Speed and Trixie are pissed that he snuck out with them and drive him home.
Later on, Speed is flown by himself to Royalton Towers. As Speed talks with Royalton inside, we see that Spritle and his chimp snuck onboard the plane so that they could divulge in the hoard of candy inside. They pass out from their candy binge. Royalton lays out several contracts for Speed to sign, but he is hesitant. He tells Royalton about watching races with Pops on TV, and how it’s not just a sport to his family. It’s like a religion, and he says that the major sponsors are like the devil to them. Respectfully, he must say no to the deal. Royalton laughs him off and gives him a lesson on the history of racing. Meanwhile, Spritle and his chimp run amok inside the building by stealing a cart and driving all over the place while hopped up on sugar. Royalton pretty much flat-out states that his company fixes a lot of races (including the Grand Prix) and threatens Speed to sign the contracts or else he’ll be sorry. Speed refuses, and so Royalton tells him that when he races next in Fuji, he guarantees that he won’t even finish the race and he’ll crush his family. Speed, Spritle, and his chimp are escorted out of the building. At the next race, sure enough, Speed gets taken out by another driver and crashes on the track.
Royalton launches a smear campaign against the Racer family, stating that they are using illegal parts on their cars. Racer X and an inspector arrive at the house to talk business. They’ve been trying to get evidence that Royalton is dirty, but have been unable to do so until now. They would like to recruit Speed to help them. Spritle asks why Racer X always wears his mask, but the inspector says that if anyone found out his true identity, they would attack him. The inspector has learned that Taejo has a file that would prove Royalton’s corruption and put him behind bars for the rest of his life. The only thing Taejo wants in return is for them to help his family business, which is going down the drain due to Royalton. There’s a cross-country race coming up soon. Taejo will only hand over the file if Racer X and Speed are on his team, and if they win (which would put his family business back into the spotlight). Pops refuses to allow Speed to enter the race, since Rex was killed in one of those races. At night, Speed and Trixie discuss how they will be able to compete in the race. They will tell Pops that they are going skiing, but instead enter the race. Trixie will be able to spot for him using her helicopter.
The cross-country race is home to the most dangerous drivers who will do anything to win. At the race, the inspector thanks Speed for agreeing to compete, and in return he has modified the Mach 5 with more devices to deflect any possible attacks. The race begins at sunrise, and the drivers are off. Many of the drivers automatically try to attack Racer X, Speed, and Taejo, but are foiled in their attempts as they drive through the desert. One of the drivers blows out Speed’s tire, but he deflates it in mid-air and continues to race. Speed, Racer X, and Taejo look out for each other and take out the other drivers. They still lose the first leg of the race though, which pisses off Taejo. As they depart for the night, Speed tells Racer X that they made a good team today, and it felt like they’ve been doing it for a while. Speed tells Trixie over dinner that he thinks Racer X might be Rex. Even though they recovered and buried a body from the crash, the body was so burnt up that it could’ve been anybody. Trixie says that Rex wouldn’t have done that to his family. A policeman knocks on the door, and shows Speed that his family has come down to see him after watching him race on TV. Pops is upset that Speed would disobey him, and tells him to pack his things. Speed stands up and says that he’s going to finish the race. Mom steps in and says that if he’s going to stay, they’re all going to stay. Even though upset, Pops goes off with Sparky to make some adjustments to the Mach 5 so that it can run smoother.
Later that night ninja assassins go after Taejo, Racer X, and Speed. They slip a liquid into Taejo’s mouth while he sleeps. Another ninja tries to knock Racer X out with a dart gun, but Racer X fights back and beats him up. Another ninja tries to inject Speed with something while he sleeps, but Spritle wakes up and screams, waking everyone else up. Sparky tries to help, but is quickly beaten up. Speed tries to defend himself, but is kicked in the face. Spritle and the chimp grab the ninja’s legs while Speed knocks the pants off of him. The ninja runs into Pops, who’s of course not happy that his family was attacked. Pops throws the ninja around the room like a rag doll and then kicks him out of the window. Taejo was drugged with something that will render him incapable of driving a car for several hours, since he can barely walk. During the next leg of the race, Trixie takes Taejo’s place on the track. Eventually, Royalton’s goons show up and hold Speed, Racer X, Trixie, Taejo, and Pops at gunpoint at a point in the track where there are no cameras. Spritle and his chimp hid in the trunk of the Mach 5 again, and they surprise the villains. Everyone fights off against the goons and they eventually get the upper hand, retrieving all the guns. Speed, Racer X, and Taejo get back in the race and wind up falling behind. They enter the cave part of the race where Rex was killed. Speed is knocked off a mountain, but hops from one side to the other and drives up a cliff. By the time he gets back in the race, Speed comes out ahead of the others. Speed, Racer X, and Taejo win the race.
It turns out though that Taejo wanted to win the race just so that his family’s stock would go up. He probably didn’t have a tape, and even if he did he wouldn’t have handed it over. Speed is furious that he entered the race for nothing and speeds off to the track at night. While driving, another car appears behind him. It’s Racer X. Speed rams his car against Racer X, and they both spin out of control. They stop on the track and Speed asks him why he’s there. He heard about what happened with Taejo, and so he came down there to calm Speed down. Speed confronts Racer X and tells him that he knows he’s actually Rex. He first showed up on the circuit two years after Rex’s death, and he drives just like him. Racer X removes his helmet/mask and reveals his true face. He says that he’s not Rex, but he knows that he would be proud of him if he was there. Speed asks him why he should continue racing, but Racer X tells him that he’ll have to figure that out on his own.
Speed goes home and packs up his belongings. Spritle and his chimp walk in and ask him where he’s going. He doesn’t know, but he just wants to get out of there. Spritle asks if they can come along, much like the way Speed asked Rex if he could go. Speed tells him no, and says that he’ll understand when it’s his turn. Speed walks towards the door when Pops asks to speak with him. He sits Speed down and tells him that he’s very proud of how he’s turned into a man. He regrets letting Rex go, and so he wants Speed to know that if he does leave, the door will always be open to him. During the heart-to-heart talk, the doorbell rings. Its Taejo’s sister, who apologizes for what happened but would like to talk to Speed. She hands him an invitation for the Grand Prix, which is in two days. Speed is overwhelmed by the invitation and accepts it. Taejo’s sister says that he won’t need luck, since he already has a nice family and he’s a great driver. After she leaves, Pops realizes that they have a problem they don’t have the Mach 5. The family, Sparky, and Trixie work furiously to make another car in time for the race. They finish making their new car and go down to the Grand Prix. Speed presents his invitation and is allowed to race, which pisses off Royalton because he fixed the race so that Cannonball Taylor would win with his company’s new car. While Speed changes in the locker room, Royalton offers the other drivers one million dollars to whoever takes Speed out of the race. As soon as the race starts, a driver causes Speed to be held back while another car tries to ram him. Speed jumps his car in the air and speeds off. When he catches up with everyone else, all the other drivers gang up on him. Speed is able to outmaneuver them and eventually catches up to Taylor at the head of the pack. Speed gets ahead, but Taylor uses an illegal device to attach his car to Speed’s and ram the hell out of him. While stuck, Speed jumps his car so that the cameras can see the illegal device Taylor is using. The jump also causes Taylor to lose control and crash. However, Speed’s car won’t start due to the damage he received.
Speed sees that the other drivers are making their way towards him and he’s a sitting duck on the track. Speed closes his eyes, feels his car, and is able to get it working again. While Speed catches up with the other drivers, he thinks about all that’s happened to him. Speed makes an explosive finish and wins the Grand Prix. The crowd goes wild for Speed’s victory except for Royalton, who’s screwed over because he’s now exposed for his cheating. It’s also revealed that Racer X really is Rex. He blew up his own car, attended his own funeral from afar, and got plastic surgery so that no one would recognize his face. Speed’s family, Sparky, and Trixie go up to the stage where Speed is and congratulate him. Speed of course grabs Trixie and gives her a big kiss in front of everyone.
The film ends with the Racer family having their picture taken with the Grand Prix trophy, and newspaper flashings showing that Royalton was sent to prison.
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