Trying to escape their dead-end life in the desert town of Perfection, Nevada, Valentine "Val" McKee and Earl Bassett find themselves sidetracked when corpses mysteriously start piling up around them. When Val and Earl have a run-in with some creepy tentacled creatures that have made lunch out of a road crew, they retreat back to the town to spread the news. Although these monsters are not exactly intellectuals, their mental powers are still mind-boggling. Detecting a human hiding in a car, they dig under the vehicle, causing it to sink into the earth. Val and Earl get help from several townspeople. With the help of Rhonda, Val and Earl kill one of the monsters. But there are three of them left, and they are each about 30 feet long. Walter starts calling the monsters graboids. When the survivors think they have outwitted the graboids by taking refuge atop the roofs of buildings, the graboids merely destroy the buildings' foundations, killing two people. The graboids are quick learners when it comes to stalking their prey, and the humans must continually be on their toes if they are to stay alive while they try to find a way to defeat the graboids.

When an army of Graboids - giant, carnivorous underground worms - threaten the Petromaya oil refinery in Mexico, its owners call on Earl Bassett, who once helped kill four of the creatures in Perfection, Nevada. Having squandered the money that came from his resulting celebrity status, Earl is convinced by the $50,000-a-head bounty offered, as well as the enthusiasm of admirer Grady Hoover, who becomes his partner. At Petromaya, Earl and Grady meet geologist Kate Reilly, and begin Graboid hunting, tricking several Graboids into swallowing bomb-rigged, remote-controlled cars. When they find they're facing a lot more Graboids than they ever expected, Earl calls his friend Burt Gummer, a survivalist who arrives well-stocked with weapons.

Tremors 3: Back to Perfection sees the original Graboid hunter, Burt Gummer (Michael Gross) return home from a wild safari in Argentina. No one has seen a Graboid in almost ten years since the monsters first surfaced in the back water town. However, the tourist site Perfection has become has sickened not only Burt but some of the other people who have been involved with the Graboids. Nancy Sterngood (Charlotte Stewart) along with her daughter Mindy (Ariana Richards) remained in town while the local store owner died and his niece, Jodi Chang (Susan Chuang) has taken over the family business while Jack Sawyer (Shawn Christian) has cashed in on the legend of Perfection. Now the Graboids have returned and with them are the Shriekers who have also evolved and are working together to eradicate every living person in Perfection. And if to make matters worse, the Government has labeled Graboids an endangered species.

In 1889, the town of Rejection, Nevada, depends on a nearby silver mine for its income. Rejection has a few residents. Christine Lord runs the local inn, which doesn't get a lot of business because Carson City is the busiest settlement in the area. Pyong Lien Chang, his wife Lu Wan Chang, and his son Fu Yien Chang are immigrants from China, and they own Chang's Market. Other residents include Old Fred, Brick Walters, Stony Walters, Big Horse Johnson, Soggy, miner Juan Pedilla, and Christine's friend Tecopa. When a hot spring causes four eggs to hatch, several men who work in the silver mine are killed by whatever hatched from the eggs. Everyone is too terrified to enter the mine. No one wants to risk their lives, even if shutting down the mine would mean the death of the town. With the mine shut down, the mine's owner, Hiram Gummer, arrives in the area from Philadelphia to investigate. Juan acts as Hiram's guide. As it turns out, each egg hatched a Graboid, but 1889 was about 100 years before they were called Graboids, so Hiram calls them Dirt Dragons. When the Dirt Dragons prove to be too much for the residents of Rejection to handle, Hiram calls upon Black Hand Kelly, the fastest gunslinger in the west. Kelly brings in the heavy artillery, which proves to be absolutely useless when the baby Dirt Dragons grow to enormous size, and one of them kills Kelly. As a result, Hiram decides to leave town, while everyone else refuses to leave the place that they call home. While Hiram is in Carson City, he intercepts a wired message that the Dirt Dragons are no longer in the area where the mine is -- they're heading right for Rejection. Hiram goes to a local firearms store, and he returns to Rejection, very heavily armed. With those weapons, which include a huge punt gun, Hiram and the townspeople prepare to defend the town against the Dirt Dragons.