Sam Daniels (Dustin Hoffman) and his wife (Rene Russo) are both working for a federal infectious disease laboratory, but their marriage is on the rocks. A mysterious lethal illness, remarkably like Ebola fever, breaks out at various sites in America, all eventually connected to a pet shop that received a monkey from an illegal smuggling operation. Most cases are immediately isolated and contained, but a town in California develops an epidemic of the new disease.

The lab is called in and the military enforces a strict quarantine that divides families and prevents anyone from leaving the area. One worker dies quickly and Sam's wife falls ill. The crass General Donald McClintock (Donald Sutherland) is convinced that the nation can be saved only by the annihilation of the town by a gigantic bomb.

A plane sets out on the gruesome mission. Meanwhile, Hoffman leaps from a helicopter onto the cargo ship where the sailor-smuggler has just died leaving a photo of the monkey carrier. Sam makes a televised appeal for help locating the cute but dangerous, little monkey; a terrified mother responds and the creature is snatched from the arms of her child.

With military snipers in hot pursuit, Sam returns to the town, radioing the baffled bomber pilots with a barrage of reasons why they should ditch their mission of destruction. He puts the tiny monkey to work producing anti-sera and vaccines, which--in only a matter of minutes!--rescue the town, his wife, and his marriage. The pilots disobey orders and dump their bomb in the sea.