Harry, Ron and Hermione's Adventures of Cars 3 is an upcoming Harry Potter/Disney Pixar crossover film to be made by Shadow101815. It will appear on Google Drive in the near future.
Lightning McQueen is preparing for a big race, when suddenly Mater visits him in his trailer, keeping him company. Lightning then asks Mater to give him some quiet. Mater leaves and McQueen soon races with his long time friends Bobby Swift and Cal Weathers. One day, a mysterious black car named Jackson Storm, who is part of a new generation of racers who use the latest technology to run faster than veteran racers, comes from nowhere and beats him. After a few more races, there are six more high-tech cars, and Bobby, Cal, and the rest of the old racers decide to retire. At the next race, Lightning has the lead, but Storm catches up and overtakes him. Lightning tries to catch up, but one of his back tires blows out, and he launches into a barrel roll, becoming badly injured.
Four months later, Lightning is recovering back in Radiator Springs but is afraid he might never be able to race again. Lightning has mostly been watching a video of the last race of his late mentor Doc Hudson. Lightning tell his girlfriend Sally that he doesn't want to be forced into retiring, like Doc, but also that he can't race like before. She tells him to try something new, and Lightning decides to start training again.
Lightning calls Rusty and Dusty, the CEOs of Rust-Eze, saying that he wants to train like Storm, and they inform him that they're opening the new Rust-eze Racing Center. Lightning travels there, only for Rusty and Dusty to tell him that in order to pay for the facility, they sold Rust-eze to a car named Sterling, who introduces him to his trainer, Cruz Ramirez, who is a big fan of him. Lightning attempts to use the simulator in the facility to train, but he shoots forward and crashes into the screen in the process. Sterling then tells McQueen in his office that his racing days are ending, and that in order to provide Sterling with revenue, he will be selling legacy merchandise instead. McQueen, annoyed that Sterling cares more about him as a brand than a racer, asks for one more race, and makes a deal with Sterling: if Lightning wins, he gets to continue racing; if he loses, he'll retire and sell Sterling's merchandise. Sterling agrees to Lightning's deal, and lets him train at Fireball Beach with the help of Cruz.
Lightning is able to train on the beach and help Cruz become experienced as well, but his top speed is 198 mph, which is still slower than Storm. Lightning sees a dirt track known as Thunder Hollow and decides to do more difficult training there. At the speedway, Lightning (disguised as Chester Whipplefilter to avoid paparazzi) comes upon the Crazy Eight Demolition Derby and he and Cruz are joined by a huge school bus named Miss Fritter. The derby starts and after Lightning challenges Miss Fritter to avoid hitting Cruz, she becomes the last undamaged car. Because of this, Cruz is declared the winner and Lightning becomes upset that he failed to win. Not only that, Cruz accidentally trips over a water truck that blasts water on McQueen, blowing his cover and revealing his identity to the crowd.
While leaving Thunder Hollow, McQueen gets angry at Cruz, telling her that instead of making progress in being faster than Storm, he is too busy taking care of her. Upon hearing this, Cruz says she wanted to become a racer her whole life because of him, but when she got to her first race, she saw that all of the other cars looked stronger, and therefore left. Cruz then leaves McQueen and Mack on her own.
Lightning sees a report on TV that Storm set a new record for the fastest lap in a race. He calls up Mater to get some good advice and he replies that Doc's trainer would know the most about racing. Knowing his trainer to be an old truck named Smokey, Lightning persuades Cruz to help look for him. Arriving at Doc's hometown, Thomasville, they come across Smokey and Lightning says he doesn't want to be forced out of the sport he loves like Doc was. Upon hearing a story of one of Doc's races, Lightning remarks that he wishes he could have seen Doc at the happiest part of his life, his racing years. This prompts Smokey to show Lightning a garage containing a whole wall of pictures of himself and Doc together. He tells Lightning that coaching him was the best part of Doc's life and that while he can never be faster than Storm, he can be smarter than him. Lightning participates in a practice race against Cruz, but he still can't catch up to her. After watching footage that shows Doc pulling ahead of another car after drafting, Cruz and Lightning are successfully able to complete the exercises. They perform a final practice race, and Lightning takes the lead down the final stretch. However, he then has a flashback to his crash and loses the race, before heading off with Cruz and Smokey to the opening Piston Cup race in Florida.
At the race, Lightning lines up in the last position due to his failure to appear at the qualifier and he passes the other racers using the skills he learned at the dirt track. However, Sterling asks Cruz to leave because she is merely a trainer, and when a wreck occurs on the track and Lightning pulls into the pits, he decides to have Cruz finish for him in order for her to have a chance at racing, remembering her story about wanting to be a racer. McQueen and his pit crew get Cruz ready to race while Mater distracts Sterling from intervening. While Cruz races, both Smokey and Lightning act as her crew chiefs. After several laps, Cruz pulls up behind the pack of racers and she picks her way between them before pulling right behind Storm on the last lap. When they approach the finish line, Storm rams her into the wall, but Cruz drives up the wall and flips in the air over Jackson in a manner similar to Doc in the racing story from earlier, thereby winning the race. Upon this victory, Sterling immediately offers Cruz to race for him, which she declines, and takes Tex's offer to become the new Dinoco racer instead. After both Lightning and Cruz are declared the winners due to each racing for part of the race, he says he will continue racing.
Back at Radiator Springs, Cruz (now as Dinoco #51) and Lightning (now as The Fabulous Lightning McQueen, to honor Doc Hudson) learn that Tex has bought Rust-eze from Sterling and taken over their sponsorships. Lightning says that he will take on the role of Cruz's new crew chief, implying that he will "continue racing" through his frequent training with Cruz. They then perform a practice race together using their newfound skills.
In a post-credits scene, Mater has trouble with his smartphone when he tries answering a call.
- Dobby, Hagrid, will guest star in this film.