Every year, all of the classic Hollywood monsters (consisting of Frankenstein's monster, his wife Repulsa, a Mummy, the Witch Sisters, Bone Jangles the Skeleton, Dr. Jackyll/Mr. Snyde, Swamp Thing, and Dragonfly) gather at Count Dracula's castle in Transylvania for the "Monster Road Rally", a road race similar to Wacky Races, awarding the winner with the "Monster of the Year" award as well as many other macabre prizes as announced by Dracula's wife and co-host, Vanna Pira. This year, however, Dracula receives a postcard from the Wolfman stating that he has retired to Florida and thus will not be participating in any more races.Dracula fears they will have to cancel the race due to this sudden absence. Luckily, Dracula's wolf-like minion Wolfgang also notifies him of a way to create a new werewolf. After searching an old book for information, it is revealed that every five centuries, the full moon comes into the perfect position to transform a human into a werewolf, on three nights in a row that begin the following night. The one next in line to become the next werewolf is revealed to be none other than Shaggy Rogers, who recently demonstrated his skills on the racetrack by winning a car race with the help of his pet dogs that serve as his pit crew, a talking Great Dane named Scooby-Doo, and Scooby's young nephew Scrappy Doo.Dracula sends his hunchbacked henchmen, "The Hunch Bunch" (consisting of the unintelligent, incomprehensible Crunch and the sly, well-articulated Brunch), to America on a mission to turn Shaggy into a werewolf, and bring him back to his castle for the race. On the first night, the Hunch Bunch attempt to cut a hole in the roof above Shaggy's bedroom to let the moon shine on him. However, Scooby learns of their plan and rescues Shaggy just in time before his transformation could begin, but fails to convince Shaggy and Scrappy of the Hunch Bunch's presence. The second night, they go after Shaggy while he is shopping with Scooby at a supermarket, but they again miss their window due to their own incompetence. On the final night, while the trio is at a drive-in movie, along with Shaggy's girlfriend Googie, the Hunch Bunch manage to expose Shaggy to moonlight by dropping the sunroof of his customized race car with a push of its button, causing Shaggy to be transformed into a werewolf.However, an unexpected anomaly cuts the Hunch Bunch's celebration short when they learn that Shaggy's hiccups are forcing him to alternate between human and werewolf. Not noticing Shaggy's transformations into a werewolf, Googie sends Shaggy to a nearby snack bar for something to cure his hiccups, and he attracts horror from the other movie watchers along the way. Scooby, hearing them speaking of a werewolf loose in the theater, hides in a nearby car. The Hunch Bunch attempts to abduct Shaggy, who flees from them, and is then chased by the crowd when they see him as a werewolf. Upon meeting Scooby and seeing his reflection, Shaggy flees the drive-in with his car, Scooby, Scrappy, and Googie in tow, escaping his pursuers with the car's customized features, losing his hiccups in the pursuit and thus remaining trapped in werewolf form. The Hunch Bunch then knocks the group out with moon dust from their vehicle, the "Bat-Copter", and fly back to Transylvania, towing the car.Upon reviving the group and cementing their predicament is indeed happening, Dracula informs Shaggy that he was turned into a werewolf in order to fill the missing slot in his monster road rally. Shaggy, having no desire to be a werewolf, is displeased with his current situation, and refuses to participate in Dracula's plans. Dracula attempts to pressure Shaggy, speaking of the pre-race party and all its rewards, showing the awards for the race, imprisoning Shaggy and his gang in a guest bedroom, and locking them in a trapped room during their attempt to escape. Ultimately, however, Shaggy still refuses, and Dracula finally offers him a bargain: if Shaggy agrees to drive in the race, and wins, Dracula will change him back to a human, and allow him and his friends to leave. The deal is made, but Dracula is still determined to double-cross Shaggy and keep him as his werewolf.The gang is then given good lodgings and treated as guests in the castle, allowed all the food they wish for breakfast. Dracula then shows them the trail that Shaggy will have to follow for the race and consents to let them navigate the track in their own racecar, with the "Werewolf Wagon" currently undergoing maintenance for Shaggy. Dracula attempts to rig the track through sending the Hunch Bunch to implement traps, but despite their efforts, Shaggy completes the course expertly, making the Count worry that he may lose his werewolf. He subsequently changes the racecourse, sabotages the Werewolf Wagon, and has the Hunch Bunch deprive Shaggy of sleep.The next morning, Googie energizes Shaggy with a kiss and he repairs the Werewolf Wagon shortly after the race begins. Throughout the entire race, everyone conspires against Shaggy and Scooby, from the Hunch Bunch's booby traps to some of the monstrous racers shrinking them or shooting lightning at them to Dracula himself putting up false detour signs and stealing their engine. But thanks to Googie and Scrappy, who follow along in their own car as their pit crew, as well as the incompetence of Dracula, the Hunch Bunch, and the racing monsters, they often end up doing more harm to themselves than him. After numerous failed attempts, Dracula loses his patience and unleashes his secret weapon, a towering ape-like beast named Genghis Kong. Genghis Kong grabs Scooby, much to his and Shaggy's horror. As the other racers near the finish line, Googie and Scrappy return and rescue Shaggy and Scooby, then both pairs work together to make the monster fall down onto the other cars, leaving an easy path to victory for Shaggy.Furious to see all of his schemes have failed, Dracula refuses to revert the spell, stating that there is no way to turn Shaggy back. However, after Vanna Pira reveals that the solution is in Dracula's spellbook, the gang steals the book and make an escape. Dracula chases after them in his weaponized car, and then his own plane after the car is destroyed in the chase. The four only barely manage to dodge Dracula's powerful gadgets, and seconds before Dracula gets the best of them, a thunderstorm ensues and Dracula's plane is struck by lightning, sending him plummeting into the ocean below where he is chased off by a shark.In the end, back home, Googie uses the book to change Shaggy back to normal. That night, the gang all sit down to watch another horror movie and eat pizza. In this final scene, Dracula and the Hunch Bunch sneak up to their window and menacingly announce their return as the film ends.