A contagious disease ravages the planet. A couple, Paul and Sarah, and their teenage son Travis are secluded in their home deep in the woods in an undisclosed location. After Sarah's father, Bud, contracts the disease, they kill him and burn his body.
That night, they capture an intruder breaking in. Paul ties him to a tree and places a bag over his head overnight to confirm he is not suffering from the disease. The stranger, Will, explains he was searching for fresh water for his wife and young son. Will offers to trade some food for water. Sarah suggests taking the strangers in, reasoning the more people, the easier it would be to defend themselves. Paul reluctantly agrees, and drives Will to collect his family. Along the way, they are ambushed by two men. Paul kills them, while accusing Will of setting him up. Will assuages Paul's mistrust.
Paul returns with Will, his wife Kim, and son Andrew. After establishing their rules, which include keeping the only entrance locked with a key Paul or Sarah wear around their neck, the two families establish a sense of normality, and grow closer.
One day, Travis's dog Stanley begins barking aggressively at an unseen presence and chases it into the woods. Travis, who had followed Stanley, insists he heard something in the woods. That night, Will seemingly contradicts a story he told Paul earlier, resulting in Paul's increasing distrust of Will.
That evening, Travis discovers Andrew sleeping on the floor of Bud's old room suffering from a nightmare. He also finds the front door of the house open. The families investigate, finding a bleeding and gravely sick Stanley on the floor. They kill and burn the dog. After Travis reveals the door was open, Sarah suggests sleepwalking Andrew opened the door, causing more distrust. Paul suspects Andrew is infected, and decides they should quarantine in their separate rooms to ensure no one is sick. That evening, Travis is awakened by a nightmare about his grandfather.
The next morning, Travis overhears Andrew crying constantly. Kim tells Will they need to leave. Travis informs his parents that he too may be infected. Paul and Sarah don protective masks and gloves, and take weapons to confront Kim and Will. Will draws a gun and takes Paul captive. He insists his family is healthy, repeatedly tells Andrew to keep his eyes shut and orders Paul to remove the mask. Will demands Paul give him "a fair share" of food and water so they can leave. Sarah and Paul overwhelm Will, and force him and his family outside. Will tries to beat Paul to death until Sarah shoots Will, killing him. Kim flees into the woods with Andrew. Paul fires at them, killing Andrew, then Kim.
Later, Travis awakens in bed, visibly sick. His mother comforts him as he dies. Paul and Sarah, now visibly infected, sit at the dinner table in silence. They share a shattered, devastated look.
