Eighteen years later, the actors in the TV show Galaxy Quest find themselves attending fan conventions after their careers failed to take off following the show’s end. Each cast member has a reason to resent their lack of success; The captain Peter Quincy Taggart (played by Tim Allen), never got another leading role, the Science officer Doctor Lazarus, (played by Alan Rickman), resents that for all his Shakespearean acting skills, he’s known as a bug-eating alien science officer on a failed show, and the communications officer Gwen DeMarco (played by Sigourney Weaver), resents that she’s dismissed (or leeringly appreciated) as the resident dumb blonde. Only the engineer, Tech Sergeant Chen (played by Tony Shaloub) seems unfazed by their lack of success. All they have left is the swarms of fans who have painstakingly followed and analyzed the TV show and are for more “into it” than the actors themselves ever were. They alternately appreciate, love or despise their fans.
Events are set in motion when the captain is approached by an alien who needs his help as captain. It turns out that this alien race have perceived Galaxy Quest almost as a documentary, and have modeled their society after the technology and ethos given on the TV show. Only Captain Peter Quincy Taggart can save them, and he follows them and does what he at first believes as a once-off acting bit.
Of course it isn’t, and Taggart has made things worse. He enlists his fellow cast-mates, who uncover to their horror how bad things really are, and how poorly equipped they are to deal with it, and the aliens certainty that the actors are their civilization’s best hope.
It is seems all a bit complicated, you’re right. The movie is a parody of a TV show which becomes a parody of itself which becomes terrifyingly real which ends in becoming the TV show again. It pushes the edge of the fourth wall again and again, with references back to science fiction fandom, Galaxy Quest and real shows, like Star Trek and Star Wars. It is an excellent movie even if you don’t understand all the tropes it lovingly spoofs, and Rickman is particularly dazzlingly as a man who despises how his audience have bought into a role he despises but comes to exemplify the heroism of his character more than anyone.