| Director | Ivan Reitman |
|---|---|
| Budget | $80 million |
| Box office | $98.4 million |
Late one night, a large meteor crashes in the Arizona desert, witnessed by aspiring firefighter Wayne Grey. The next morning, college professors Ira Kane and Harry Block investigate the crash site, discovering that the meteor has landed in a cave and secretes a strange blue liquid. Upon further study, they find that the meteor harbors extraterrestrial microorganisms that evolve rapidly, condensing millions of years of evolution into mere hours.
Over the next few days, the organisms evolve into fungi, flatworms, and aquatic creatures, despite being unable to breathe oxygen. As the United States Army takes control of the site, Ira and Harry attempt to continue their research with the help of Brigadier General Russell Woodman and epidemiologist Dr. Allison Reed. However, the Army soon shuts them out, prompting Ira to go to court, where he admits that he was discharged from the Army after an anthrax vaccine experiment went wrong, causing severe side effects in 140,000 soldiers.
Frustrated by the Army's takeover, Ira and Harry infiltrate the base to collect a new sample, only to find that the cave now harbors a full-fledged alien ecosystem, complete with tropical plants and animals. The situation escalates when a large reptilian creature kills the owner of a local country club, and other alien creatures begin attacking residents of nearby areas. The creatures, though unable to survive in Earth's oxygen-rich environment, are rapidly evolving to adapt.
As the alien attacks become public, Governor Lewis demands answers, and Reed calculates that the alien organisms could overrun the U.S. within two months. Despite opposition, Woodman authorizes a napalm strike to destroy the meteor and its surrounding ecosystem. However, Ira realizes that heat accelerates the aliens' evolution, and that the napalm strike will only cause further problems.
In a desperate attempt to destroy the ecosystem, Ira theorizes that selenium, toxic to nitrogen-based life forms, could kill the aliens. With the help of his students, Ira discovers that selenium sulfide, the active ingredient in Head & Shoulders shampoo, could be their solution. Armed with a fire truck filled with shampoo, the team heads to the ecosystem to carry out their plan, but the army initiates the napalm strike early, fusing the alien life into a gigantic amoeba-like creature. Regardless, the team injects the creature with the shampoo via an enema on its central hole just as it begins to multiply into two, causing it to implode.
With the alien threat neutralized, Governor Lewis declares Ira, Harry, Wayne, and Allison as heroes. Wayne is finally made a full firefighter, and Ira and Allison begin a romantic relationship, while the town returns to normal.