Deep in the Klatu Nebula, the brave Thermians have been trying to survive the advances of the genocidal Sarris, leader of a gang of Fatu-Krey soldiers that almost resemble humanoid dragons. Sarris destroyed their home planet and killed billions of Thermians, until only a few remained on a ship called the NSEA Protector, a ship built to the exact specifications of another ship seen on "historical documents" beames into space from Planet Earth. What the Termians did not know was that the documents were really a TV show called Galaxy Quest, in which Jason Nesmith (Tim Allen) plays Commander Peter Quincy Taggart, head officer onboard the Protector. Faced with extinction, the Thermians sought out Jason and his "crew" (the other main actors) for help against Sarris.
After torturing and killing the Thermians' former commander, Sarris discovered the existence of an experimental device called the Omega 13, and wanted to know what it did. Some believed it to be a device that would destroy the universe in 13 seconds, others believed that it was a 13 second time jump into the past.
When Sarris discovered that the "crew" were nothing more than a group of actors, he attempted to blow up the Protector with a core implosion, release Nesmith's team into space, and suffocate the Thermians. All three failed, however, as the actors put up a fight, killing many of Sarris's minions before blowing up his ship with mines from a mine field.
However, Sarris was able to escape his ship by using a device to teleport onto the Protector. Disguised as Fred, he wounded or killed most of the crew (ironically excluding Guy, who always thought he was the expendable character with no last name), but Jason activated the Omega 13, reversing time. He attacked Sarris, who later tried to stab them with a knife but was beaten with a cane by Mathasar, the leader of the Thermians.
After the actors returned to Earth, arriving at a stage on a Galaxy Quest convention, Sarris reappeared, having survived the previous encounter, but Jason shot him with an Ion Nebulizer (blaster pistol), completely disintegrating him, much to the applause of the audience.