In Crater Lake, Northern California, Dr. Richard Calkins is informed by his colleague Dan Turner that he and his girlfriend Susan Patterson have made an incredible discovery in a nearby cave system. The three head down and discover a system of cave drawings, including what appears to be a depiction of people fighting off a Plesiosaurus, thus providing evidence that dinosaurs existed at the same time as humans did. However, a flaming meteorite crashes into the lake just overhead, resulting in a cave-in that destroys the cave system and the drawings, while the three scientists are barely able to escape alive. The local sheriff, Steve Hanson, sees the meteorite crash and radios in the incident before continuing on his patrol.
Several months later, Sheriff Hanson meets with the three scientists to go search for the meteorite. Turner and Patterson dive down to the bottom of the lake, only to find out that the meteorite is still too hot to recover and has resulted in the entire lake becoming significantly warmer than before, rising to approximately ninety degrees. Somewhere else on the lake, a birdwatcher is setting up his equipment when the monster suddenly rises out of the water, moves onto the shore, and kills him.
Two friends, Arnie Chabot and Mitch Kowalski, running low on money, decide to start a boat rental service. Their first customer is U.S. senator Jack Fuller, who rents a rowboat for a quick fishing trip for $20. However, he is attacked and killed by the monster. Arnie and Mitch see the empty boat drifting in the middle of the lake and go out to retrieve it, finding only some large blood stains inside the boat. They bring the boat back to shore as evidence for the Sheriff. Then the sheriff finds many dead animals, and takes the case.
Some time later, a performer named Ross Conway and his wife Paula are on their way to a show when their car suddenly begins to break down. They stop at a gas station and learn from the mechanic that their car won't be repaired for several more days. The attendant tells them that the fastest method of transportation at this point is by boat across the lake. The couple heads down to Arnie and Mitch's dock to rent a motorboat for $25 and head out. While out on the lake, they are attacked by the monster, but manage to outrun it due to the boat's motor and run it aground. When the monster pursues them onto the shore, Ross empties the can of gasoline into the boat and sets it ablaze, fending off the monster.
Arnie and Mitch, as they walk away from renting the boat out to the couple, begin to argue about their boat-renting service. Mitch claims that he is tired of being bossed around by Arnie, and the two eventually fight. Their scuffle leads to the water, where the two discover the severed head of Fuller floating in the lake just as the Sheriff arrives. As he takes in the head for evidence, he orders them to stay away from the lake, and to not use any more boats. Realizing that the couple from earlier is still out there, Arnie and Mitch head out in another boat to search for them. They eventually discover the charred remains of the motorboat and the distraught couple, both too mortified to explain what happened to them. The couple is taken away in an ambulance, and the Sheriff issues a stern warning for Arnie and Mitch to not head back out onto the lake.
At the local diner, the sheriff spots a man who is wanted for armed robbery in the nearest town. During the crime, the clerk and another customer were killed. The sheriff pursues the suspect into the forest. After the suspect drives his car off a cliff and jumps out, the sheriff pursues him on foot. The chase eventually leads them down to the shore, where the Sheriff shoots the suspect in the knee, before stopping to hide behind a tree and reload his weapon. During the brief pause, the monster snatches the suspect, dragging him under the water. The sheriff does not hear the attack happen, but he discovers a large blood stain on a nearby rock. Meanwhile, Calkins's autopsy report comes in, and the coroner notifies the sheriff that the wounds were caused by an animal's teeth and that the attacking animal is not only of a significant size, but also lives in the lake.
When the sheriff returns the next day to the location where the robbery suspect went missing, he finds several massive footprints before the monster suddenly emerges. He fires all six shots in his revolver at it, before jumping into his car and driving away. He tells Calkins, Turner, and Patterson about the incident, and his description of the monster fits that of a plesiosaurus. While the three scientists are excited at the idea of a living dinosaur in the lake, the sheriff is determined to kill it before more lives are put at risk.
The sheriff, Calkins, Turner, and Patterson host a town meeting in the diner the next day, informing the town of the danger and what they plan to do to stop the monster. Arnie and Mitch ultimately take the scientists' side in favor of keeping the monster alive, saying it'll bring in a significant amount of money for the town. However, a man named Ferguson is attacked by the monster and barely manages to make it to safety inside the diner. The Sheriff, Turner, Patterson, Arnie, and Mitch all head outside to confront the monster, which is just outside the barricade of farming vehicles and a wall of hay bails. The Sheriff starts up a bulldozer, but Arnie attempts to stop him at gunpoint, saying that the monster must live. The Sheriff convinces him that nothing will stop the monster without killing it, and Arnie jumps in the back, shotgun at the ready. As the monster draws closer, Arnie panics and attempts to flee, only to be caught and killed by the monster. The Sheriff slams into the monster with the bulldozer, causing it to drop Arnie's corpse. When it reaches its head down to try to pick up Arnie's body again, the Sheriff drives the bulldozer forward and repeatedly slams into the monster's neck, finally killing it.
In the aftermath of the battle, the Sheriff, Calkins, Turner, Patterson, and Mitch all mourn Arnie's death, with Mitch vowing to continue the boat rental service that he and Arnie started, softly repeating "our boats...our boats."
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