David Levinson (Jeff Goldblum) realizes that the only way to defeat the aliens shields is to give it a cold (a computer virus) that would disrupt them all long enough for the Earth ships to fight back. He and Cpt. Steve Hiller (Will Smith) fly one of the crashed alien ships back tot he mothership, and infect it with the virus before blasting back home. On Earth, once the shields are down, Russell (Randy Quaid) sacrifices himself to destory the last ship threatening the last US military base.
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