In Crater Lake, Northern California, Dr. Richard Calkins [Bob Hyman] is informed by his colleague Dan Turner [Richard Garrison] that he and his girlfriend Susan Patterson [Kacey Cobb] have made an incredible discovery in a nearby cave system. The three descend and are shown a web of cave drawings, including what appears to be a depiction of people fighting a Plesiosaurus, suggesting that dinosaurs coexisted with humans. A flaming meteorite crashes into the lake overhead, triggering a cave-in that destroys the cave system and the drawings, while the three scientists barely escape with their lives. The local sheriff, Sheriff Steve Hanson [Richard Cardella], witnesses the meteor crash and radios about the incident before continuing his patrol.
Months later, Sheriff Hanson [Richard Cardella] reunites with the scientists to search for the meteorite. Turner [Richard Garrison] and Patterson [Kacey Cobb] dive to the bottom of the lake, only to learn that the meteorite is still too hot to recover and has caused the entire lake to warm significantly, reaching around ninety degrees. Somewhere else on the lake, a birdwatcher [Hal Scharn] sets up his equipment when the monster suddenly rises from the water, crosses the shore, and kills him.
Two friends, Arnie Chabot [Glen Roberts] and Mitch Kowalski [Mark Siegel], strapped for cash, decide to start a boat rental service. Their first customer is U.S. Senator Jack Fuller [Marc Eliot], who rents a rowboat for a quick fishing trip for $20—but he is attacked and killed by the monster. Arnie and Mitch glimpse the empty boat drifting in the middle of the lake and go out to retrieve it, finding only dark bloodstains inside. They haul the boat back to shore as evidence, and the Sheriff starts digging into the rising casualties.
Some time later, Ross Conway [Michael F. Hoover] and his wife Paula Conway [Suzanne Lewis] are on their way to a show when their car suddenly malfunctions. The mechanic warns that repairs will take days, so the attendant suggests they rent a boat from Arnie and Mitch for a while. The couple head to the dock, pay $25 for a motorboat, and venture out onto the lake. They are attacked by the monster but manage to outrun it and run the boat aground. When the monster pursues them onto the shore, Ross dumps gasoline into the boat and sets it on fire, fending off the creature.
Back at the dock, Arnie and Mitch resume their argument about their business. Mitch, fed up with being bossed around, accuses Arnie, and their tangle spills into the water. They soon discover the severed head of Fuller floating in the lake, which the Sheriff collects as evidence. He warns them to stay away from the lake and to avoid any more boats. Realizing the couple is still out there, Arnie and Mitch take another boat to search for them and eventually find the charred remains of the motorboat and the shaken couple, too dazed to explain what happened. The couple is taken away in an ambulance, and the Sheriff sternly orders Arnie and Mitch to stay away from the water.
At the local diner, the Sheriff spots a man wanted for armed robbery in the nearby town. During the confrontation, the clerk and a female customer are killed. The Sheriff pursues the suspect into the forest. After the suspect drives his car off a cliff and jumps out, the Sheriff chases him on foot to the shore, where he fires six shots in the knee from his revolver, only for the monster to snatch the suspect and drag him under the water. The Sheriff, unaware of the attack, later spots a large blood stain on a rock. Meanwhile, Calkins’s autopsy reports reveal that the wounds were made by an animal with teeth from a creature of significant size that lives in the lake.
The next day, the Sheriff returns to the robbery location and finds giant footprints before the monster emerges. He shoots again and drives away, recounting the incident to Calkins [Bob Hyman], Turner [Richard Garrison], and Patterson [Kacey Cobb], describing a plesiosaur-like killer. The three scientists share a cautious enthusiasm for the possibility of a living dinosaur, but the Sheriff remains focused on stopping the threat before more lives are at risk.
A town meeting is held in the diner to lay out the danger and the plan to stop the beast. Arnie [Glen Roberts] and Mitch [Mark Siegel] back the scientists, arguing that keeping the monster alive could bring substantial money to the town. Yet Ferguson [Joe Sasway], a diner patron, is attacked by the monster and barely escapes into the diner's safety. The Sheriff, Turner [Richard Garrison], Patterson [Kacey Cobb], Arnie [Glen Roberts], and Mitch [Mark Siegel] confront the creature near a cluster of farming vehicles and a hay bale wall. The Sheriff starts a bulldozer, but Arnie tries to stop him at gunpoint, arguing that the monster should not be killed. Arnie jumps aboard the vehicle, shotgun in hand, but when the monster draws closer, panic overcomes him and he is seized and killed. The Sheriff drives the bulldozer straight into the monster’s neck, repeatedly ramming it and finally ending the threat.
In the aftermath, the Sheriff, Calkins [Bob Hyman], Turner [Richard Garrison], Patterson [Kacey Cobb], and Mitch [Mark Siegel] mourn Arnie’s death, and Mitch vows to carry on the boat rental venture that he and Arnie started, softly repeating, > our boats...our boats.
