We open with a narration and a sweeping view of a river wash. A couple, Pat Wheeler (Grady Vaughn) and his girlfriend, Liz Humphries (Yolanda Salas) are sitting in his car when something pushes the car over and down a ravine and into the wash. A giant claw fills the screen and credits roll.
A group of teens are dancing at a soda shop. Gordy (Don Flournoy) and his girlfriend, Jennie (Jan McLendon) pull up outside in Gordy's roadster. Chase Winstead (Don Sullivan) and his girlfriend Lisa (Lisa Simone) arrive in Chase's roadster. Chase inquires about Pat and Liz, and when told their car may have broken down, Chase immediately challenges that by saying, "not in his heap, I worked on it myself." Lisa works for the Wheeler family. She is an exchange student from France and Mr. Wheeler is her sponsor. She tells everyone Pat didn't come home for dinner and his father was very upset. Old Man Harris (Shug Fisher) arrives in his 1932 Ford. Chase wants to buy his car but Harris is not interested in selling. The gang of kids leaves for the drive in.
The Sheriff (Fred Graham) pulls up to the large Wheeler home. He meets with Mr. Wheeler (Bob Thompson) regarding his missing son, Pat. Wheeler tells the sheriff that both his son and Pat's girlfriend didn't come home last night. The sheriff infers they may have eloped. Wheeler blames Chase; he thinks Chase has too much influence over his son and the rest of the kids in town. The Sheriff reminds Wheeler that Chase is a good kid and works hard after his Dad died "on one of your drill rigs." The Sheriff stops at Compton's Garage to talk to Chase. He thinks Chase may know where Pat and Liz have gone. Chase tells him he doesn't know, but dismisses the elopement theory. The final stop by the Sheriff is the Humphries' farm. He meets with Ed Humphries (Howard Ware) and his wife Agatha (Stormy Meadows) about their missing daughter, Liz. They have no idea where she is, but are more gracious and understanding of the Sheriff's challenge than Mr. Wheeler. On his way out he bumps into Harris. He knows Harris is a drinker and asks to smell his breath. Satisfied, he sends him on his way. No sooner has the Sheriff walked away than Harris pulls out a bottle and takes a swig. He drives off.
Mr. Compton (Cecil Hunt), the owner of the garage where Chase works, returns with a load of four quarts of nitroglycerin. Wheeler ordered it in case there was a blowout and oil fire. Compton tells Chase he took the nitro out of the safety cases. Chase is horrified and immediately puts them in the shed in safety cases. His Dad taught him how to handle this dangerous liquid. When the phone rings, Compton reminds Chase that it is not for them, "It's not our ring." But Chase recognizes it as the Sheriff's ring code and eavesdrops--if it is an accident, Chase figures to get a tow job out of it. His hunch pays off, and Chase heads off in his roadster to the scene, with Compton following a short time later with the tow truck.
When the Sheriff arrives at the accident scene, Chase is already there. They examine the wrecked vehicle and notice the engine is still warm. Chase directs the Sheriff's attention to the skid marks on the road. They go at a direct right angle to the direction of travel. They also notice blood on the upholstery. While waiting for the tow truck, Chase tells the Sheriff about his little sister's need for leg braces.
While Compton is driving down the road in the tow truck he passes a man on the highway with a suitcase. Lurking in the underbrush is the Giant Gila Monster, hereinafter GGM. The man (Desmond Doogh) pulls out a pack of cigarettes and lights one. He hears something, but can't see source of the noise. The GGM approaches and the man falls back into the brush. His suitcase sits untouched. As Compton hooks up the wreck with Chase assisting, the Sheriff tells them the car was reported stolen. On his way home Chase notices the suitcase on the side of the road and stops. The Sheriff pulls up almost immediately. He speculates that it might belong to the person who stole and wrecked the car. They look around and see a loose cigarette and half a pack. They put the case in the Sheriff's car and both depart. Chase stops at the Wheeler home to see Lisa. She is upset. Mr. Wheeler has demanded she stop seeing Chase. He blames Chase for Pat's disappearance.
Chase is driving the tow truck when a brand new Cadillac passes him at great speed. The driver notices the GGM on the road ahead and veers to avoid it crashing into a fence and disabling his car. Chase stops to render assistance and notices the driver is unhurt but very drunk. At the wheel is Horatio Alger "Steamroller" Smith (Ken Knox). He is dressed in a tux and introduces himself and explains that a large pink and black thing blocked the road. Chase hooks up his car and tows it back to the garage. The next morning Chase is hammering out the fender of Smith's car and singing. It wakes Smith, who is sporting a hangover, but he likes what hes hearing. He gives Chase his card and tells him to "look me up." He tips Chase $40 then departs.
The Sheriff pulls up to the garage and notices Chase working on the wrecked car. It seems Chase has helped himself to a couple of new tires off the wreck for his roadster. Chase tells the Sheriff he will help with the search for Pat and Liz. The Sheriff notices the switch of tires, but doesn't object.
Gordy and Chase and their girlfriends search the wash. The GGM watches them. Chase and Lisa walk down the ravine and along the wash. Chase notices marks that look like something large was dragged. Chase tastes some water in the stream, but spits it out and remarks that it tastes bitter, and strongly of minerals. Gordy honks his car horn, and Chase and Lisa run back. Gordy tells Chase he found Pat's car at the bottom of the wash about two or three miles back by the old reservoir. Gordy and Chase pull it out with the tow truck. They bring the wrecked car back to the garage. The Sheriff arrives and asks where they found the car. He notices the same kind of damage as other vehicles sustained. When the Sheriff asks about Compton, the garage owner, Chase tells him he made a fuel oil delivery. On the road, Compton is driving a fuel oil truck. The GGM knocks it off the road and the wreck explodes into flames. Chase closes up the garage and heads home. His mother, (Gay McLendon) meets him on the porch. She blocks his path into the house, so he picks her up. She makes him close his eyes as he enters the house--she has a surprise for him. His little sister, Missy (Janice Stone) is wearing leg braces. Missy exclaims, "Look what Lisa got me." She makes an attempt to cross the room, but falls, gets up and walks a few more steps. Chase then sings her a song. Chase thanks Lisa for the braces. He intended to buy them the next day, but Lisa beat him to it. The phone rings. The Sheriff is inquiring about Compton as there was an accident involving a fuel truck reported. Chase meets him at the garage. Harris reported the accident. Chase, Harris and the Sheriff leave in Chase's car, as the Sheriff's car has a flat tire. They arrive at the accident scene and the truck is still burning, but no trace of Compton. The Sheriff tells Chase that livestock have been reported missing.
At the garage, Gordy and his friend hear a radio report about Steamroller Smith agreeing to DJ at their upcoming barn dance. They are waiting for Chase, who drives up with a new paint job on his roadster.
Harris is driving down the road, singing and drinking. He sees a train and decides to race it to the crossing. The train misses. Further down the line a trestle is damaged as the GGM slides under it. The train derails and many of the passengers are killed and eaten. After witnessing the derailment, he turns around and drives back to the Sheriff station and makes a report. The Sheriff interviews Harris. Dissatisfied with the story and suspecting drunk driving, he locks him up.
Chase's mom arrives home in Chase's roadster. Missy is inside with Chase; shes carrying a suitcase and informs Chase shes spending the night at the Blackwells. The phone rings; its the Sheriff again. He asks for a book on reptiles. At the Sheriff's office he and Chase discuss the possibility of some animal developing gigantism. Harris claims to have seen the GGM, as have some of the survivors of the train wreck. Chase recounts his story of Steamroller Smith's encounter with what he described as a large pink and black creature.
At the barn dance people are arriving. Inside there is music and dancing. Chase introduces Steamroller Smith. Smith plays a record. Mr. Wheeler and the Sheriff meet at the garage. Wheeler is mad because his son Pat's car was moved without a proper investigation. He accuses him of protecting Chase. Wheeler concludes his son is dead; he also notices the stolen tires and demands the Sheriff arrest Chase. They drive over to the dance to make the arrest. Smith plays a record and asks the audience to identify the singer. Lisa guesses that it is Chase. Chase picks up a banjo and sings. The Sheriff and Wheeler arrive in separate vehicles, and the GGM simultaneously makes its way to the barn. The GGM crashes through the wall. Panic ensues. The Sheriff retrieves his shotgun from the car and fires with little effect, but does manage to chase it off. The Sheriff deputizes Wheeler and orders him to keep everyone there. Chase and Lisa drive off. Chase has a plan. Chase races back to the garage to retrieve the nitroglycerin, all four quarts. He races along the road and notices a break in the fence and the damaged Blackwell residence, where his little sister was visiting. They drive across a field and spot the Blackwell family and Missy fleeing. Missy falls and is covered by Lisa while Chase races towards the GGM. He jumps clear of his roadster just before it hits the creature. It explodes on impact killing the GGM. The Sheriff arrives and asks, "What did you hit him with?" Chase responds, "my brand new, 100% completed hot rod." Wheeler tells the Sheriff with Compton gone, Chase is unemployed. He tells the Sheriff he has a job for Chase.