In a secret military laboratory hidden inside a facility posing as a pesticide manufacturer, a sealed tube breaks, triggering the release of a virulent bacterium. Dr. Nielsen becomes a central figure as the outbreak unfolds, but the immediate chaos is sparked when the plant’s security officer, Joanie Morse, activates Protocol One and seals everyone inside. The workers initially believe the lockdown is a simple pump malfunction, and two coworkers—Tom Schmidt and Bob—restart a pump in a bid to restore operations, unaware that they are already contaminated.
As fear spreads, Cal Morse, a County Sheriff and Joanie’s husband, seeks medical help and contacts Dr. Dan Fairchild, a former employee who once developed an antidote. The mission is joined by Vic Flint, Bob’s father, who supplies support to the rescue effort. The United States government’s Accident Containment Team (USA CT), led by Major Connolly, arrives on the scene and imposes quarantine measures, while a public cover story about contaminated yeast is broadcast to reduce panic. A rescue team is dispatched to administer the antidote to the workers who have been exposed.
When the weapon is released, the workers sanitize the lab, destroy animal tests, and inoculate themselves with a still-to-be-tested antidote. The USACT team tracks down Dr. Nielsen and his colleagues, only to discover the lab floor empty and the air lock smashed from inside. A power outage is traced to Dr. Ramesh Kapoor, a P4 worker who damages a power box in a desperate bid to protect his findings. A subgroup of workers—Bob, Tom Schmidt, and Tippett—believes they remain unaffected and attempts to leave, but they are gradually pulled into the contagion as the bypassed air circulates the contaminated atmosphere and Schmidt’s contact lens spreads the pathogen throughout the building. Meanwhile, Joanie destroys a paper containing a deactivation code but recalls it; however, the code is invalidated once the USACT taps into the system.
The rescue team eventually confronts the quarantined group and orders them to stay put; Tippett resists and is killed, while the others are confined to a room to await inoculation as the team retreats to regroup. Fairchild leads the team through an unoccupied service conduit toward the P4 lab, hoping to reach the antidote, but they encounter Dr. Nielsen in distress and retreat, only to be ambushed by the P4 workers. Dr. Kapoor murders one of the assailants, and the team watches in horror as the group of infected workers overpowers them, even after inoculation—the antidote fails to stop the spread, and the infected become homicidal.
With the rescue team incapacitated, USACT declares Protocol Two, leaving the building to endure the weapon’s deadly effects while the staff are sanitized. Cal urges Fairchild to help retrieve a working antidote and save Joanie. Inside the facility, Cal encounters the infected Bob and must shoot him to prevent further harm. Most workers fall to the infection, but Joanie remains unaffected. Schmidt persuades Joanie to accompany him to the P4 lab to secure the antidote, and along the way they collide with Dr. Nielsen who wants to suppress the incident and with Dr. Kapoor who aims to kill them. They escape, though Schmidt is seized by the infection as they reach safety.
In the P4 lab, Schmidt succumbs to the illness while the P4 workers close in on them. Joanie retrieves the antidote and escapes, while Schmidt attacks the intruders and breaks Kapoor’s neck before being killed by the others. Joanie eventually reunites with Cal and Fairchild, and the trio fends off the pursuing infected workers who manage to tear Fairchild’s biosuit. They enter the P4 lab again to discover why Joanie remains unaffected and why the antidote failed. A test reveals that Joanie’s blood is loaded with estrogen, progesterone, and antibodies—she is pregnant—which explains her immunity.
As intruders attempt to reach and overwhelm the trio, booby traps ignite parts of the building, and one worker rips open Cal’s suit, only to be killed by Joanie. Before he is rendered incapacitated, Fairchild drafts a new antidote recipe into a computer, based on Joanie’s condition; the plan includes thorazine to sedate patients during recovery. The trio tests the new antidote on Fairchild, and it proves effective.
Armed with injection guns, Cal and Fairchild inoculate the infected workers, while Joanie carries a batch of the antidote to the building’s decontamination system. Nielsen refuses to be injected and returns to the lab, where he ultimately commits suicide upon recognizing that the plan has failed. Joanie uses the decontamination system to disseminate the new antidote throughout the building, eradicating the weapon and treating the survivors. She then deactivates the quarantine, and Fairchild inoculates Cal. USACT evacuates the victims, retrieves the dead, and seals the building, bringing the crisis to a somber close.
Throughout the ordeal, the relationships among the key figures—Cal Morse, Joanie Morse, Fairchild, Nielsen, Kapoor, [Katz] and others—are tested by fear, loyalty, and the race to discover a cure, with the fate of every character hanging on the balance of science, strategy, and survival.
