While transporting toxic chemicals, a pair of truckers strike a deer and lose one of their canisters. It rolls into a river, floats downstream, and splashes several beavers at their dam.
Later, college students Mary, Zoe, and Jenn arrive at Mary's cousin's cabin near the dam. Although Zoe and Jenn are distraught that there is no cell phone coverage, Mary insists that they spend the next two days without interruptions from technology or boys. While swimming at the lake, the girls discover the beaver dam. When Jenn swims closer to see a beaver, they are surprised by a bear. Smyth, a local hunter, scares off the bear and admonishes the girls to stay away from the beavers and dress more appropriately.
At night, the girls' boyfriends – Buck, Tommy, and Sam – arrive and play a prank on them. Annoyed, Mary orders them to leave, but Zoe, who knew they were coming, insists they stay. Jenn talks Mary into letting them stay, as she wants to resolve her relationship issues with Sam, who has cheated on her. As the couples separate, Jenn leaves to take a shower, only to be attacked by a beaver. Tommy kills it with a baseball bat, but Jenn and Mary feel something is wrong with it beyond rabies. They put the corpse in a bag and put it outside on the porch. In the morning, they discover it is missing, and Jenn insists that it was not merely eaten by wildlife.
All but Jenn go swimming in the lake. Sam and Mary discuss whether to tell Jenn about their affair, but they are interrupted when beavers chew off Buck's foot and scratch Tommy. The same beaver that attacked Jenn earlier chases her back to the house. She pins it to the counter with a knife, but not before it scratches her leg. Sam throws Zoe's dog into the water as a distraction, and the rest of the group flee to the house, where they realize the beavers have cut the phone lines. As zombie beavers surround the house, Tommy offers to leave with Buck to get help, and Zoe accompanies him. Their drive is interrupted as the beavers bring down a tree in the road, and Tommy sets off on foot, only to be crushed by another falling tree.
Smyth rescues Buck and Zoe, and they drive in his pickup truck to return to the house, which has been boarded up by the others. Unable to enter it, they go to the neighbors' house. They discover the neighbors are dead, and their phone lines cut. As a result of her wounds, Jenn turns into a beaver-like zombie and attacks Mary. As Jenn's saliva falls in Mary's mouth, Sam rescues Mary by hitting Jenn from behind. Mary and Sam lock themselves in the bathroom and check each other for scratches. Satisfied that neither is wounded, they begin to have sex, but Jenn interrupts them and kills Sam by biting off his penis. Meanwhile, Buck turns into a zombie and attacks Smyth. Zoe escapes by jumping through a window, bloodying herself in the process.
As a beaver accidentally sets Mary's cousin's house on fire, Sam rises as a zombie. Once Mary kills Sam, she reunites with Zoe. Smyth, now zombified, shoots at them as they flee in his truck. Jenn jumps on top of the truck, but Zoe runs her over. Together, Mary and Zoe reach the spot where Tommy died and turned into a zombie. With the road blocked, they decide to walk. Believing that Zoe may be bitten due to her bloody appearance, Mary holds her at gunpoint with a pistol recovered from Smyth's truck. As Zoe protests that she is not infected, Mary begins changing into a zombie. Zoe kills her with an axe and walks away. On the road, she encounters the same truckers who had earlier dropped the canister, and they run over her accidentally. In a post-credits sequence, a honeybee becomes infected from a corpse and returns to its hive in a zombified state.
