*CUT TO THE CHASE*
NOTE: This spoiler was submitted by Emilio.
In the opening scene, a couple of truckers are transporting canisters of a biohazard substance belonging to the MAMARONECK Medical Research Facility. They accidentally hit a deer on the road. The impact of the collision causes one of the biohazard canisters to fall into the river. The canister floats down the river where it eventually comes to rest in a beaver dam. The canister starts leaking a green gas.
Mary (Rachel Melvin), Zoe (Cortney Palm) and Jen (Lexi Atkins) travel to the town of Ashwood for the weekend. The three plan on staying at Mary's cousin's house. Jen has a lot on her mind as she is recovering from the revelation that her boyfriend, Sam (Hutch Dano), has cheated on her. Mary and Zoe planned the trip as a way of helping Jen recover from it.
The house that they are staying at is facing a lake that happens to have a beaver dam. The river that the canister fell into to is right next to the lake. The river and lake is separated by an isthmus. Jen and Zoe soon realize that they cannot get any cell phone service, but Mary tells them that there is a land line connection in case anything there is any trouble.
The girls decide to go for a swim in the lake. While they’re swimming, they come across the beaver dam. Jen wants to see a beaver, so she goes towards the dam, and notices another beaver dam (which is where the canister arrived) in the river. Mary comes to the conclusion that the same beaver built both dams. The girls come across a bear, but before they are harmed, they are rescued by a man named Smith (Rex Linn). Smith is a redneck who warns them to stay away from the beavers, and dress more appropriate.
That night, Tommy (Jake Weary), Mary's boyfriend, Buck (Peter Gilroy), Zoe's boyfriend, and Sam surprise the girls at the house. Reluctantly, Mary invites the guys inside after Jen says she doesn’t mind that Sam is with them. Tommy and Mary leave to go have sex. Buck and Zoe go to have sex in their bedroom. Jen goes to the bathroom to take a shower. In the shower, she is attacked by a beaver. After hearing Jen’s screams, Tommy comes in and kills the beaver with a baseball bat. Jen and Mary believe something is wrong with the beaver because it did not have pupils. Later on, it is shown that the beaver survived.
The next day, the entire group goes out to swim in the lake. Jen is still scared from what happened with her and the beaver in the shower. Because of that, she does not join them. At this point, it is shown that Sam had cheated on Jen with Mary. Everybody, other than Tommy and Jen, knows about it. The beavers then attack Buck and bite his foot off. They also manage to scratch Tommy in the process. Jen gets attacked by the same beaver that attacked her the previous night in the shower. Jen stabs the beaver with a knife, but not before it scratches her leg. The beavers damage the land line connection. So, now the group has no way to communicate with the outside.
Zombie beavers (Zombeavers) then start to surround the house. Buck has lost a lot of blood due to the loss of his foot. Realizing that Buck will not make it through the night, Tommy decides to take him to the hospital with Mary's car. Zoe goes along with them. In order to stop them, the beavers bring down a tree to block the road. Tommy gets out and goes on foot to find some help. He is killed when a tree follows on top of him, due to the beavers. Smith comes to the rescue and agrees to help Zoe and Buck get back to the house in his truck.
At the house, Mary, Sam, and Jen seal all the windows and the door with wood, in order to keep the beavers out. Unfortunately, the seal prevents them from letting Zoe, Smith and Buck in when they come back. Zoe and her group go to their neighbor, Winston Gregorson's (Brent Briscoe) house, where he lives with his wife Myrne (Phyllis Katz). When they get there, they find both of the Gregorsons dead. Their land line has also been destroyed by the beavers.
At the other house, Jen begins to turn into a beaver-like zombie, because she was scratched. She grows a tail and her teeth begin to grow out to the length of a beaver’s. She attacks Mary, and some of her spit falls into Mary's mouth. Sam rescues Mary by hitting Jen from behind. They lock themselves inside a room. Mary begins to piece together that anybody that gets bit or scratched by the zombeavers turn into these beaver-zombie hybrids
At the Gregorsons' house, Buck turns into one of the zombies and kills Smith. He attacks Zoe. Zoe locks herself in the Gregorsons' bedroom. She is then attacked by the zombified Myrne. She escapes by jumping out the window. However, when she lands, she becomes unconscious. Meanwhile, a beaver sets the other house on fire.
Zoe returns to help Mary in Smith's truck. Smith is now a zombie and starts shooting at his truck. Zoe and Mary manage to survive. Jen attempts to jump on to the top of the truck, but Zoe runs her over. Mary and Zoe reach the spot where Tommy died: the road blocked by a tree. They decide to get out and walk. Mary starts to believe that Zoe is bitten. She points the gun at Zoe, but instead starts to turn into a zombie. Zoe then kills her with an axe and walks away. On the road, she sees the same truckers who had dropped the canister, and they accidentally run her over.
In a post-credits scene, a bee is seen on one of zombie's carcasses. It flies away and returns to the hive in a zombified state.
A couple of truckers accidentally hit a deer and, inadvertently, release a barrel containing a biohazard chemical into a local river. The chemical causes beavers to become more aggressive. Three girls, Jen, Mary and Zoe, arrive at a house, which is located right across from where the chemical was released. They are joined later by their three male friends. The next day, they are attacked by the zombie beavers (zombeavers). Anyone who is scratched by these beavers begin to change into a zombie beaver themselves. In the end, Zoe and Mary escape, but when Mary begins to change into a zombie, Zoe kills her. Zoe reaches the road and tries to flag down a passing truck, being driven by the same truckers who released the chemical. They accidentally run Zoe over.
In a post-credits scene, a bee is seen by one of the zombie carcasses. It flies away and returns to its hive in a zombified state.