At a sci-fi convention, the cast of a long-cancelled television series "Galaxy Quest" is going through the motions of meeting their fans, even though the show has been off the air for years, and the actors are having a difficult time finding other work due to "type-casting." They squabble, each remembering personal quirks and on-set peccadilloes, either jostling for the limelight or just "doing the gig," and by happenstance, find themselves portraying their characters in a real outer-space adventure, persuaded by desperately seeking aliens who have caught transmissions of the show as it zipped through space at light-speed, and not only liked what they saw...they bought it hook, line, and beridium sphere.
It's a fun conceit, combining the background stories of actors with the characters they're playing put into a situation where their attributes as actors inform their actions in a real-life approximation of the roles they play. it stands to reason that the traits they embody to play the roles would come to the fore when reality sets in—it implies, of course, that they could do all this without "handlers," make-up assistants, or even (ya know...) decent writers and directors. It's fun to imagine. And the writers of the film have the "Star Trek" types down: the blow-hard lead actor (Tim Allen) who looks after himself until the chips are down; the second-in-command alien character—a fan-favorite—portrayed by an actor (Alan Rickman) who aspires to more prestigious roles and makes efforts to distance himself from his most popular one—until the role's inherent dignity and importance occurs to him; the actress (Sigourney Weaver, paralleling both Lt. Uhura and her "Ripley" character in Alien) trying to rise above the inherent sexism of her role, opening "hailing frequencies"; and the "also-ran's" (Tony Shalhoub and Sam Rockwell) who are stuck in their careers, forever typecast and unable to move on, bitterly watching their co-stars take better advantage.
There's also a tip of the hat to the faithful fans who start out as convention geeks, then play a pivotal role in the survival of the crew and their mission—and they don't even have to do it with a write-in campaign to the network.
Fiction becomes fact becomes fiction becomes fact; Galaxy Quest is an inter-dimensional trek on a Universe that's folded in on itself, through all the frontiers influenced by TOS and its progeny that explored new frontiers on a journey that seems on-going. It's also one of the best examples of a rare film genre that usually comes off as lame, childish, or patronizing—the science fiction comedy.