Caution: potential spoilers.
First of all, I just wanted to mention, I remember seeing this movie in the theater in 1999, and I believe the theater was called the "Galaxy 6." I just always found it mildly amusing that "galaxy" was in the name of the movie as well as the theater at which I first saw it. Meh. I'm just lame that way...
Anyway, it starts out with a scene from a campy old sci-fi TV show called "Galaxy Quest" (in the Star Trek vein, but clearly not nearly as good; at least it seems less campy than Lost in Space). The scene, as it turns out, is being played at a convention for fans of the show (17 years after it was cancelled). The cast are waiting to be introduced so they can come out on stage, but... the star of the show, Jason Nesmith (Tim Allen), who had played Commander Peter Quincy Taggart... is late. Clearly, the rest of the cast deeply resent Nesmith, and for good reason. They also don't seem that happy about their own roles on the show, which they know wasn't as good as the fans seem to think it was. There's Gwen DeMarco (Sigourney Weaver), who had played computer officer Lt. Tawny Madison; her role was mainly just a sex symbol, which of course she resents (all she had to do was repeat whatever the computer said). There's Alexander Dane (Alan Rickman), who had played an alien named Dr. Lazarus; being a Shakespearean actor, he resented having to play a role he considered beneath him, and is sick of his character's catch phrase ("By Grabthar's hammer... you shall be avenged!"). There's Fred Kwan (Tony Shalhoub), who had played tech sergeant Chen; he actually seems pretty laid back about... well, everything, and he's probably my favorite character in the movie. Then there's Tommy Webber (Daryl Mitchell), who had played pilot Lt. Laredo (Corbin Bleu), apparently a child prodigy (a la Star Trek: TNG's Wes Crusher), though of course he's grown up, now; he mainly seems to resent Jason always acting like he's more important than the rest of the cast. But then, they all resent that.
Anyway, Jason eventually shows up, and as usual, steals the show. Oh, and the guy who was the announcer or whatever, at the convention, was Guy Fleegman, who had appeared as an unnamed crewman in one episode, and got killed off (as often happened to such characters on "Star Trek"). Well, at the convention, Jason is approached by some aliens... who he assumes are fans in costumes, naturally, and from what they say, he further assumes that they are arranging an appearance of some sort for him. Later, he overhears people talking about what a laughingstock he is, so he's upset for the remainder of the convention, and that night he gets drunk.
The next morning, the aliens (called Thermians) show up. Their leader is Mathesar (Enrico Colantoni), whose manner of speech is really annoying, IMO. (The DVD extras talk about how he inspired the other Thermian actors' method of speaking, though luckily, none of them sound at all like him, as far as I can tell.) The others are Laliari (Missi Pyle), Teb, and Lahnk. While Jason naps in the limo he instructed them to get, they take him to their ship, the "Protector 2" (named after the fictional ship from the show). They want him to negotiate on their behalf with General Sarris, an alien warlord who has caused the Thermians great suffering. He wants the Omega 13, a fictitious device mentioned in the series, but which the Thermians have created (as the entire ship is based on the supposed technology of the original "Protector"). Jason assumes all this is some little acting gig (though he does appreciate the production values). He breezes his way through it, instructing them to fire on Sarris, then leaving. It's not until the Thermians return him home using a sort of transporter that engulfs him in a pod of some kind of goo and shoots him through space (and a black hole, apparently), that he finally realizes it's all real. (I should note that later in the movie, we see a device that's more like a Star Trek transporter than the pods are.)
He excitedly shows up at a store opening, where his castmates are making an appearance, to tell them about the experience, but of course they don't believe him. But the Thermians show up again, still wanting "Taggart" to negotiate with Sarris (who had survived the attack Jason had ordered), and he asks the others to come along. They eventually go along, thinking he's talking about an acting gig, and are shocked to find it's real. The only one who automatically takes it in stride is Fred (who seems to enjoy himself; actually he basically seems to be stoned). Jason is excited to be in space for real (and I find it odd that, though he only went ahead of them by maybe a few minutes, he suddenly knows a lot more about what's going on than he did before). Everyone else wants to leave (especially Guy, who worries for much of the movie that he'll get killed like his character did). But it's too late, Sarris is there, so everyone has to play their parts.
Well, there's a bit of a battle between the Protector and Sarris's ship. There's an excursion to an alien planet with some other dangerous creatures. There's more trouble when Sarris's people board the Protector and attempt to kill all the Thermians, and also set the ship's self-destruct. But Jason gets some help from a fan named Brandon, who had picked up a vox (communicator) that Jason had dropped back on Earth, previously. He and his friends knew all about the schematics of the Protector and theories about what Omega 13 actually was, and since the Thermians had based everything precisely on the TV show, that knowledge came in handy. I should also mention that there was a Thermian named Quellek, who idolized Dr. Lazarus. And Laliari and Fred got quite close. And finally, there's another space battle (though honestly, the "battles" are little more than the Protector being chased and fired upon by Sarris's ship).
I don't really want to say anything more about the plot, or how it all ends (either the final space battle or any of the few scenes that come after it), though I will say I quite liked the ending. I loved the whole movie, actually. It was really funny, particularly if you're a fan of any of the type of shows being parodied here. And there was some decent character development for the actors (mainly Jason and Alexander, though I suppose they all had insecurities to get over), as well as for Mathesar. ...One thing that always rather bothered me was no one doing an adequate job of explaining to the Thermians the difference between "lying" and "fiction." The whole point of the movie is that they thought the TV show was a collection of "historical documents," since they have no fiction on their planet, and have only recently become aware of the concept of lying, through Sarris's deceptions. But of course, fiction isn't really deception, because no one's meant to believe it's real. But, whatever... the scene where Sarris learns the truth about Jason and the others was priceless. And I think at the very least, Laliari will eventually learn what fiction is. And... yeah, I guess that's all I have to say.