An unknown viral outbreak has devastated the area of the United States. A family of three, Ray (Alexander Skarsgård), Claire (Andrea Riseborough) and their daughter Zoe (Emily Alyn Lind), have taken refuge in an abandoned fallout shelter shortly after the catastrophe to hide from the monsters outside. Zoe wakes one night after having a nightmare of the 'Breathers'. The family has difficulty coping in their cramped environment but manage by having a set of rules, such as always keeping calm and never leaving the shelter. The three soon discover at dinner that their already dwindling rations are also being consumed by a rat and try to find and kill it. The family also discovers that the rat managed to sneak into the shelter through another opening other than the main barricaded hatch. They find a small air duct but Ray reassures Zoe that the Breathers wouldn't be able to fit through the duct. They believe they may be the only ones left alive. The Breathers have been actively hunting down the family, and have nearly located them multiple times. Claire and Ray's priority is to keep Zoe alive, regardless of what happens to them.