Twenty some years after the sci-fi TV show "Galaxy Quest" last aired, its popularity seemingly hasn't waned one bit. While that's good news for those who run the related conventions across the country, the core group of actors who trek from one such gathering of "Questarians" to the next are disillusioned and getting tired of the routine.
There's Gwen DeMarco (SIGOURNEY WEAVER), who played the buxom Lt. Tawny Madison, Fred Kwan (TONY SHALHOUB) who played Tech Sergeant Chen, Tommy Webber (DARYL MITCHELL) who played Lt. Laredo as the boy wonder navigator, and Alexander Dane (ALAN RICKMAN), a Shakespearean actor who embodied the half-human, half-alien, Dr. Lazarus.
While they generally get along, they collectively despise their former "leader," Jason Nesmith (TIM ALLEN) who played the gung-ho but melodramatic Commander Peter Quincy Taggart. While Nesmith enjoys playing up to the legion of fans who still love and admire his character, some overheard remarks about being a has-been puts him into a tailspin that sends him to the bottle and ends with him being awakened the next morning with a hangover on his living room floor.
It's there that a quartet of apparent show fanatics, fronted by Mathesar (ENRICO COLANTONI), approach Nesmith once again after having met him the day before. Believing them to be escorts to his next "Quest" gig, the bleary-eyed actor accompanies the foursome, who turn out to be Thermians from some distant galaxy, onto their ship. There, he nonchalantly engages in space battle with their arch-nemesis, Sarris (ROBIN SACHS), a towering and malicious, bug-like villain intent on ridding the universe of all Thermians.
Nesmith believes all of this to be some sort of impressive fan-related production, but when he's literally sent flying back through space to Earth, he suddenly realizes it was real. Of course his perturbed co-stars don't believe a word of his story, but he eventually convinces them, despite their inherent disbelief and mistrust, to return to space with him.
Once there, and accompanied by Guy Fleegman (SAM ROCKWELL), a convention host and one-time extra on the show, the actors learn that the Thermians long-ago intercepted TV signals of "Galaxy Quest," believing them to be historical documents regarding real characters and real events.
Fashioning their society after that featured in the show, and even building a spacecraft, the Protector, similarly based on the special effect model, the Thermians have recruited Taggart and his crew in hopes that they'll help defeat Sarris and his marauding forces. Eventually realizing the scope and danger of that request, the crew of actors decides to do what they can to defend the Thermians and defeat Sarris and his troops.