As nuclear explosions ripple through New York City, the residents of a crowded apartment block rush downstairs, only to be funneled into the building’s basement as further blasts shake the structure. Eight people manage to squeeze into the shelter before the superintendent, Mickey, seals the door: Eva and her boyfriend Sam, Josh and his brother Adrien, Josh’s friend Bobby, Marilyn and her daughter Wendi, and Delvin. Inside, the cramped space becomes a small, tense world where Mickey asserts control over the group, and his authority irks Josh, Bobby, and Delvin.
The shelter’s fragile calm fractures when the door is breached from the outside by armed soldiers in biohazard suits. The soldiers’ language is indecipherable, their loyalties unclear, and they attack the occupants, snatching Wendi away. Delvin and Mickey fight back, killing three of the intruders and enabling Mickey to seize a rifle. In a bid to rescue Wendi, Josh volunteers to don a dead soldier’s suit and step outside.
Outside, Josh discovers a sealed landscape of plastic-sheet tunnels connected to a laboratory. Wearing the suit allows him to wander into a stark, clinical world where he finds several unconscious children—Wendi among them—stored in stasis units, their hair shaved and eyes bandaged. When one of the soldiers verifies Josh’s disguise by pulling out his breathing apparatus, Josh is left gasping for air and dashes back to the shelter, killing two soldiers on his way. The soldiers then weld the shelter door shut from the outside, sealing everyone inside.
Time drifts by as radiation sickness takes its toll. Josh admits to Adrien that his outside excursion was not a rescue mission for the two of them, but a desperate bid to find safety. Eva, already navigating a fragile relationship with the milquetoast Sam, grows closer to Adrien. In the shelter, Bobby volunteers to deal with the corpses of the dead soldiers so their bodies can be dumped into the septic system, while Delvin grows increasingly suspicious that Mickey is hoarding resources. Marilyn, believing Wendi is dead, enters into a sexual relationship with Bobby.
Tensions escalate when Marilyn hints that Eva should sleep with Josh, warning that the men will demand sex. As resources dwindle, anger toward Mickey deepens, culminating when Delvin discovers Mickey’s locked safe room and threatens to force him to open it. A struggle erupts and Mickey shoots Delvin in the head with the rifle. The group rejects Mickey’s defense of self-defense, and Josh and Bobby, now reckless and hairless from radiation sickness, torture Mickey to learn the code to the safe room. Eva, aiming to stop Josh, throws the rifle into the toilet. Seizing control, Josh and Bobby turn increasingly abusive toward Marilyn, while Eva and Sam fail to protect her.
Josh makes his ambitions explicit: he wants Eva, and Sam falters under the weight of the group’s violence. Eva and Sam try to shield Marilyn, but their efforts prove futile. Tasked with watching Mickey, Eva learns there is another exit— through the septic tank—to the sewer, a fact that could upend their fragile shelter. She attempts to retrieve the gun but is blocked by Bobby and Josh; she discovers Marilyn’s corpse, the evidence of a brutal struggle. Realizing she is also contaminated, Eva sees that the shelter’s protections are compromised by the broken door and the harsh outside world.
Eva sends Sam to fetch the gun, while she sabotages the power to distract Bobby and lures Josh with a false promise of sex. Josh grows suspicious and attempts to rape Eva, who fights back with Adrien’s help. When Sam finally retrieves the gun, the room erupts in a violent scramble: he shoots Adrien, possibly driven by jealousy over Eva’s feelings for Adrien. Josh beats Sam, and Bobby turns on Josh, readying to shoot him—only for Eva to slit Bobby’s throat with a tin can lid. Eva frees Mickey, who shoots Josh, ending his reign of terror. In a final act of desperation, Josh sets himself ablaze after smashing an oil lamp, igniting a fire that consumes the shelter.
Eva makes a perilous exit, donning the biohazard suit and locking herself in the safe room as Mickey and Sam remain trapped within the burning shelter. She crawls through the sewage-tank breach into the septic system, descends to the street, and emerges into a city that lies in ruins. The ending leaves the meaning open to interpretation, a stark meditation on survival, power, and the cost of shelter in a world that has been shattered.
