The prologue established in Western Australia depicts the crash of Skylab debris in 1979 and a subsequent outbreak decades later when a farmer disturbs the dormant Cordyceps novus. Pentagon operative Robert Quinn and biochemist Dr. Hero Martins arrive on scene. When the acid-like fungus breaches Dr. Martins's suit, she feels the organism seizing her motor neural pathways and shoots herself in a brief moment of lucidity. Quinn and military forces incinerate the town and store the live sample in Sublevel 4 beneath Atchison, Kansas.
Decades later, the military abandons the bunker and leases the surface to a commercial self-storage facility. A chiller coolant leak in Sublevel 4 warms the vault, waking the pathogen. The fungus infects a feral cat and later an errant deer, turning them into swollen, spore-spewing bio-weapons navigating the elevators and hallways.
As facility client Ma Rooney is pulled into wild corridor firefights, Quinn receives an automated biosecurity alarm and arrives heavily armed. Realizing the automated thermal failsafes cannot be reset from the console and that the biomass is climbing through ventilation shafts, Travis and Naomi use local industrial tools, cold containment tanks, and controlled incendiaries. Together, they seal the mutated core organism inside a pressurized containment vault, purging it with extreme cold and fire to prevent atmospheric venting.
At sunrise, Travis and Naomi emerge with their bond cemented after saving the world on minimum wage, while Quinn oversees the permanent decommissioning of the facility.