A large meteor crashes in the barren Arizona desert late at night, which aspiring firefighter Wayne Grey witnesses. The next day, science professors Ira Kane and Harry Block from nearby Glen Canyon Community College investigate the crash site, discovering the meteor has landed in a large cave and 'bleeds' a strange blue liquid when scraped.
The professors quickly learn that the meteor harbors extra-terrestrial nitrogen-based microorganisms that condense millions of years of evolution within a matter of hours. The next day, the microscopic organisms have evolved into asexual fungi and flatworms that cannot yet breathe oxygen, and aquatic life the following day.
The U.S. Army soon seals off the site, so Ira and Harry ask Brigadier General Russell Woodman, Ira's former boss, and the clumsy Dr. Allison Reed, senior researcher in epidemiology at the CDC, to aid in their research. Their efforts fail when, two weeks later, Ira goes to court to be allowed to be part of the federal investigation.
Allison questions Ira on the stand, where he reveals he was discharged from the army in the summer of 1997 after he conducted an experiment in which he developed an anthrax vaccine and then administered it to nearly 140,000 soldiers. The side effects were severe, resulting in his dismissal from his position as a top-level researcher at USAMRIID.
Upon returning to the lab, Ira and Harry discover that Woodman has stolen their research, so they infiltrate the base in disguise to get another meteor sample. They discover that the caverns now harbor an alien rainforest teeming with tropical plant and animal life, including flying insects and carnivorous plants.
That night, a large reptilian creature fatally mauls the owner of a local country club where Wayne works, and the next day a dog-sized frog-like animal attacks two older women in their home. Ira, Harry and Wayne find a valley behind the home filled with suffocating dragon-like creatures, which they theorize cannot yet breathe oxygen and are escaping from the meteor site through the local caverns. However, a newly-born dragon quickly adapts to the earth's oxygen and terrorizes a shopping mall before the trio gun it down.
With the media becoming increasingly aware of the alien attacks, Governor Lewis visits the site demanding answers, furious at not being informed earlier about the situation. Allison calculates that the aliens will engulf the US within two months, so Woodman suggests a napalm strike to destroy the meteor's contents and the surrounding town, although Lewis opposes bombing the community. Primate-like creatures suddenly attack the base and wound several members, persuading him to approve Woodman's napalm strike.
Disgusted, Allison quits the CDC and joins Ira's crew. Ira later realizes that intense heat triggers the aliens' DNA and that the initial impact to Earth activated its evolution. Allison tries to warn Woodman but he ignores Allison's pleas so the town begins evacuating for the impending bombing strike.
Looking at the positions of nitrogen and carbon on the periodic table, Ira theorizes that selenium might be as toxic to the aliens as arsenic is to Earth's carbon life. Rather surprisingly, two of his dumbest students, Deke and Danny Donald, recall that selenium sulfide is the active ingredient in Head & Shoulders hair shampoo, which they decide to use against the alien organisms.
Wayne procures a firetruck and fills it with the shampoo with help from Ira's college students. However, Woodman's napalm strike goes off ahead of schedule before the team can try their plan, triggering the entire alien ecosystem to fuse together into a single immense amoeba-like blob that stands hundreds of feet tall.
The giant mass begins multiplying through mitosis, which it would do infinitely until the country was overtaken by thousands of these gigantic creatures. The team maneuvers their firetruck underneath the mass and discovers a rectal-like orifice to spray the shampoo up into; moments before the creature splits into two, the team succeeds and the monster explodes. The governor declares Ira, Harry, Wayne and Allison heroes; Wayne is made a fully-credentialed firefighter, and Ira and Allison begin dating.
Later, Harry, Ira and Wayne are shown promoting Head & Shoulders for both hair-care and fighting aliens.
