After an unknown outbreak devastates the United States, a family of three—Ray, Claire, and their daughter Zoe—takes refuge in an abandoned fallout shelter as the Air Force begins bombing their town. The trio clings to a strict set of rules designed to keep them alive: stay calm, never leave the shelter, and keep each other safe at all costs. Yet danger stalks them from beyond the hatch.
In the claustrophobic silence, a rat discovers the dwindling food supply and becomes a tense, unwelcome challenge. The family works to kill it, only to learn the rodent found its way in through a small opening other than the barricaded hatch. They discover a narrow air duct and, in a moment of stubborn reassurance, Ray tells Zoe that the Breathers—the nightmarish figures outside—won’t be able to squeeze through. For a while, it seems they might be the last people left alive.
The threat from the outside intensifies as the Breathers close in on the shelter. Claire ends up killing the rat, but a careless accident sends a lamp tumbling and bursting into flames. The smoke rises up the duct, creeping toward the shelter and slowly exposing their position. When the smoke deposits ash on the hatch, their hiding place becomes visible to any lurking danger.
Desperation grows, and the family decides to venture to the surface to cover the shelter’s opening. Zoe, armed with a homemade periscope, spots a distant humanoid figure, and against the rulebook, she climbs through the barricade to warn her parents. They manage to retreat back inside, but the encounter leaves them vulnerable as a Breather follows.
A flashback reveals the outbreak’s chilling dawn: Ray, Claire, and Zoe race along a highway, only to be halted by a quarantine checkpoint manned by the CDC. The world outside is collapsing, and signs of containment multiply as the family witnesses the beginning of a grim new reality.
Back in the present, the Breathers grow bolder and more numerous, ultimately breaking through the shelter hatch. Claire and Zoe flee to the air duct, while Ray is captured and disappears, his fate marked by a spray of blood. On the highway, they are recaptured by American soldiers wearing night-vision gear and re-breathers, who surround them with rifles drawn.
Another flashback shows two military jets roaring overhead as they bomb Kingsville. The family hurries to an abandoned shelter, where Claire injures herself on rusted metal. The injury accelerates the realization that the virus is airborne and they are already infected, a truth that makes every breath a risk.
In the present, soldiers test Claire and Zoe’s blood, interrogating them for nearby survivors before preparing to execute them. Out of nowhere, Ray reappears, killing one soldier and incapacitating several more with startling speed and strength as he is shot repeatedly and falls. Claire and Zoe break free from their net, and, in a fierce display of drive, Claire kills more soldiers while Zoe—in a furious, protective state—exposes a soldier to the air by removing his re-breather, infecting him. The two of them barely hold back from harming each other as Claire reminds Zoe to keep calm and adhere to their rules.
Ray’s collapse signals the end of their immediate struggle, and the trio seeks shelter in a sewer system. They huddle in the damp tunnels, listening for any approaching threat, until a familiar voice answers their calls: Joey, Zoe’s neighbor and friend. He has survived and leads them to others who endure in the underground network. The group chooses to join this growing community of infected survivors as dawn breaks.
As the sun rises, Zoe notes that they’ve endured day 302. Claire offers a different perspective, reframing the math of survival: Not days, but miracles. The two climb toward a sewer hatch and watch the world outside brighten through a window, with Claire’s eyes bloodshot and unyielding as she exits into the rising light with Zoe, ready to face whatever comes next in this shattered new world.
