The film begins with Paul having to take his father-in-law out into the woods to kill him as he's been infected with some unknown disease. He and his son Travis then burn the body. We learn that Paul and his wife Sarah and their son Travis are attempting to protect themselves from this strange virus that is infecting people. They live in their home in the woods far away from the populated cities with their dog Stanley.
The following night someone has broken into the house, a lone man who nearly shoots Paul with a shotgun, which he says was an accident. They tie him to a tree outside until Paul decides to talk with him a day later. The man' name is Will and he is searching for water for his wife and small son. Paul agrees to take Will to the home where they are staying in order to get back to his wife and child. On the way they are encountered by two men who shoot at them. Paul shoots them both before they can get information from them.
Paul returns home with Will and his family, Kim and Andrew. They've brought their animals with them for food in exchange for letting them stay in Paul and Sarah's home. Paul establishes the rules of the house and the families live together and begin to become closer. Soon after Stanley hears something in the woods while out with Travis, Paul and Will. The dog chases after it and then is silenced, he's been killed but no one sees it. Paul is upset that Travis would sprint after the dog without a thought of how dangerous it could be.
That evening Paul decides to share a glass of whiskey with Will. During their conversation, Will seems to trip up his story. He told Paul earlier that his family lived with his brother, but Will says he doesn't have a brother and that it was Kim's brother he was talking about. That he had been like a real brother to him. This concerns Paul and that same night Travis awakens from a nightmare to find Andrew in another bedroom and lying on the floor, having had a nightmare himself. Travis returns him to his parents room and then finds the front door is open, then there is a loud noise. Travis goes to get his dad who comes out to find Stanley, the dog dead in the front of the house. They burn Stanley's body, and then they all sit down to figure out what happened.
Travis tells everyone he found Andrew in another room and the front door open, and he didn't open it. Paul decides that the families need to stay away from each other and so they do. But Travis later overhears Will and Kim talking, saying they need to leave, and Travis believes that Andrew is sick. Travis tells his parents this and they decide to confront Will and Kim. But, Will won't open the door and when Paul finally comes in, Will puts a gun to his head and demands they let them go. Then Sarah puts a gun on Will until they agree to put their weapons down. While this is happening, Paul subdues Will and takes his gun, and Kim runs out of the house with Andrew.
Everyone else then goes outside, and in a struggle Will overtakes Paul and is beating him. He is ready to crush his head in with a rock when Sarah shoots him. Kim takes off running and Paul fires a shot which kills Andrew in her arms. She screams at Paul to kill her, and he does.
We see that Travis' nose is dripping with blood. He eventually is seen waking up with his mother beside him. He's been infected, and eventually dies. Paul and Sarah sit at the table alone.
