In the African jungle of 1967, a deadly virus named Motaba is discovered, prompting U.S. Army officers Donald McClintock and William Ford to eliminate the site where it was found to ensure the virus remains a viable biological weapon. Fast forward to 1995, when the virus reemerges in Zaire. Enter Colonel Sam Daniels, a virologist from USAMRIID, who is dispatched to investigate this alarming resurgence.
Daniels, aided by Lieutenant Colonel Casey Schuler and rookie Major Salt, quickly learns more about the virus and returns to the U.S. He urges General Ford to issue a public health alert, but Ford dismisses the idea, insisting that the virus is unlikely to pose a threat. Meanwhile, Daniels' ex-wife, Roberta Keough, who now leads a similar team at the CDC in Atlanta, is persuaded to recommend an alert, but her superior hesitates.
The real danger arises when a white-headed capuchin monkey, the host animal of the virus, is smuggled into the U.S. by an employee named Jimbo Scott. Jimbo gets infected through contact with the monkey, which he attempts to sell on the black market. After transferring the infection to several others, the situation quickly spirals out of control, with a significant outbreak occurring in Cedar Creek, California. As the virus spreads, hospital tests reveal a new, highly contagious strain.
Amid the chaos, the U.S. Army enforces martial law in Cedar Creek, while Daniels and his team work tirelessly against Ford's directive to locate the source of the virus. A series of unfortunate accidents leads to further infections, including that of Schuler and Keough, jeopardizing their efforts. Meanwhile, Ford reveals to Daniels that a military operation (dubbed Operation Clean Sweep) is in place to bomb Cedar Creek to contain the virus and cover up its existence.
Determined to prove that a cure can be found, Daniels and Salt escape military custody, ultimately tracking down the host animal. After successfully capturing it, they learn that the military plans to bomb the town, believing it to be the only solution. In a tense confrontation involving General McClintock and the crew of the bomber plane, Daniels manages to persuade them that there is hope for a cure.
In a pivotal moment, the pilot of the bombing plane opts to detonate the bomb over water instead of the town, a decision that saves countless lives. Subsequently, McClintock's malicious actions come to light, leading to his arrest by Ford. At last, Daniels and Keough reconcile, and the citizens of Cedar Creek receive treatment to recover from the outbreak, showcasing the power of cooperation in the face of a crisis.
