The movie begins in 1987, where a young Les Daniels, who is terrified of snakes, is forced to go to a wedding where venomous snakes are passed around. When he is supposed to give one of the snakes to his father, he hesitates, giving the snake enough time to bite his father's artery. He dies in a matter of minutes.
Twenty years later, Les is still scared of snakes, while his older brother Duff teases him about it. To help him get over his phobia, Duff goes to a Native American snake proprietor, Screaming Hawk. While there, Hawk tells him about a tiny snake living inside a jar, on his desk. It's called Altucka, an ancient snake that grows at a massive rate, and killed many of Hawk's tribe. They finally were able to kill all but one of them, the same one in the jar. He is given three rules to follow: "don't let it out of the jar, don't let it eat anything living, and never fear the heart of the snake." He is told he can't have Altucka, but steals it anyway. When at home Duff accidentally breaks the jar. Les sees the snake double in size, almost instantaneously, but Duff doesn't believe him. That night, while Duff sleeps, the snake eats the pet cat. It's now grown to almost 20 times its original size. It sneaks out into the chicken coop, killing all but one of them, which it soon finishes off. When Les' mother hears the commotion, she goes outside to see what's going on, only to meet the same fate. Duff becomes scared that the legend's true, and asks some snake killers how to stop it. They say that if the snake hasn't yet reached full size, he could kill it by stabbing it in the head. He does so and seems to have killed the snake. But that night however, the snake revives and kills and eats him and the family dog.
Les returns home, after a long night of drinking, only to find the place ransacked. He calls his girlfriend, Erin, who's a police officer, as well as her ex-boyfriend, Bo, the chief of police. When Les says that he just knows that his family is dead, Bo becomes suspicious that he might be the killer and arrests him. Altucka then attacks a vacationing family, in the woods, killing and eating them. Erin starts to believe Les' story, but Bo still doesn't. Erin heads deeper into the woods and finds a giant snake skin, meaning that the creature is still growing. She helps Les escape, and they head over to Screaming Hawk to try to find out more about Altucka. Bo and his team finally head out to kill the snake. Altucka jumps them with only Bo able to escape. Before he dies, he comes to the homes of the snake catchers that Duff talked to earlier. They try to kill the snake, but they end up both getting killed.
Meanwhile, Les, Erin, and Screaming Hawk all try to find the snake to kill it. Altucka, who's by now 70 feet long, heads to the county fair, where Feedback is making a guest appearance. The snake kills a young couple and three stoned men, before making its way inside. Altucka then kills multiple people, along with one of the police officers. Feedback is able to hold the snake off long enough to save some people, before fleeing himself. On the way into the park, the trio sees a little girl trapped in a ride. Erin is able to save her, but ends up being eaten herself. Just then, Erin calls Les, over walkie-talkie. They head into a haunted rollercoaster ride, to save Erin before she is digested. Screaming Hawk is wounded trying to fight the snake, leaving Les to kill Altucka from the inside. He lets himself get eaten whole, and then cuts the snake's heart out, killing it and saving Erin.
A few months later, Les and Erin are married, with their first child, and Les has finally gotten over his fear of snakes.