Carol (NICOLE KIDMAN) is a Washington, D.C. psychiatrist, divorced from her Center for Disease Control (CDC) official husband Tucker (JEREMY NORTHAM) and raising her little boy Oliver (JACKSON BOND) on her own. Called to the scene when a space shuttle crashes to earth, Tucker is afflicted with a mysterious goo.
In the days following, the CDC declares a "flu epidemic," encouraging citizens to be inoculated at government-approved stations, but Carol suspects it is something else when a patient, Wendy (VERONICA CARTWRIGHT), reports tearfully, "My husband is not my husband."
The infected folks style themselves as peacefully conformist, and background TV reports confirm that the world's wars and nuclear threats have subsided. Though Carol admits she has treated patients with medication to help them to "fit in," now she insists on the importance of being individuals, even in the face of conflict.
Carol is helped in her effort by her colleagues Ben (DANIEL CRAIG) and Steven (JEFFREY WRIGHT), who look into microscopes and deem the invading entity "highly resilient," able to survive 700 degrees of heat. As they race to find a cure, Tucker tries to infect Oliver, and Carol follows them to Baltimore, where she is pursued by packs of aliens. In order to save her son, she turns into an action hero, stealing and crashing cars, shooting guns, and battling those who are infected.