The film begins in Leadville, Colorado. Trevor and his girlfriend, Nina, are in an abandoned building having a romantic encounter. They then head back to their respective parents' homes. They see a long line of cars at a military roadblock sealing off the city. An angry man yells at the officers because his child is sick and needs medical care. There has been an outbreak and medical help is needed throughout town. They are introduced to Captain Rhodes and Corporal Sarah Cross. Soon after, privates Bud Crain and Salazar show up and Bud immediately falls in love with Corporal Cross. Cross takes Bud to go with her while she visits her sick mother. When she arrives, Sarah talks to her brother (Trevor) and checks in on her mother. She then goes to see Trevor's friend, Kyle, after she learns his symptoms are similar to her mother's.
À Seattle, un groupe d'avocats attend une télé-connexion avec l'Australie pour une conférence en duplex. Bientôt, le signal est établi. Soudainement, les hommes de loi sont pris de foudroyants malaises. La mort est quasi instantanée. Nick Baldwin, un scientifique, intervient quelque temps plus tard auprès de policiers afin de les aider à maîtriser un forcené. Le dénommé Tarrance, devenu fou, raconte à Baldwin l'agonie des malheureux. Une fois arrivé à la morgue pour examiner les corps, Baldwin en arrive à la conclusion qu'un virus pourrait être la cause du décès. L'épidémie menace.