Galaxy Quest was once a popular "geek" TV show that eventually got cancelled but was so popular among a a cult group that it has spawned 18 annual conventions where fans gather, often in costumes based on the shows, to watch old episodes, meet cast members and actors who played even bit parts on single episodes. At Galaxy Quest 18, hosted by Guy Fleegman (Sam Rockwell), an actor who played in episodes 81 and died by the first commercial, the fans are seeing a "lost" episode which is part one of a two parter dealing with the Omega 13 device. As this episode is screening in the main room back stage sits the original cast and crew:
* Alexander Dane (Alan Rickman), who played half human/half alien Dr. Lazarus and moans about how he was once a great actor on the English stage.
*The beautiful and buxom Gwen DeMarco who plays communications officer Lt. Tawny Madison (Sigourney Weaver), and whose job on the ship was to repeat what the computer said, complains that her TV guide article was all about her bustline and how it fit into her uniform.
* Tommy Webber (Daryl Mitchell), the child actor who has since grown up and found no life other than Galaxy Quest conventions remarks about how the commander and star of the show Jason Nesmith (Tim Allen) is more than an hour late.
Nesmith finally shows up and gets into the "been there done that" groove asking if Dane has already done his "I was once an actor" thing. This, plus the fact the Nesmith is about to do a solo gig without the rest of the cast almost has everyone walking out of the dressing room and heading for home, as Jason tries to explain the gig is a couple of kids with cardboard in a garage and is nothing major.
Out on the convention floor Guy now starts introducing the crew of Galaxy Quest, who snap into their roles, especially Dane after Nesmith says the magic words to convince him that "The show must go on!" They go outside to the waiting fans, the spotlights and the roar of the crowds to sit and sign autographs and give the fans a taste of the dream.
Gwen happens to smile at Nesmith, basically because Tommy told a hysterically funny and true crack about the fans loving Nesmith almost as much as Nesmith loves himself. In that brief moment Nesmith see's Gwen and remembers times that once were and hopes for a rekindling of old on the set romance, which Gwen promptly shoots down when fans ask he if she and the commander once had a thing and Gwen replies, as Nesmith comes up behind her. Nope!
Gwen is ushered off by officials to do a "look a-like" Tawny Madison and fans in costume photo shoot. Nesmith tries to follow and is confronted by Mathesar (Enrico Colantoni), an actual alien come to Earth to seek the help of Commander Peter Quincy Taggert, the character played on Galaxy Quest by Nesmith. Nesmith thinks this is the group of kids who are paying him to do an appearance in their garage tomorrow and tells them to keep things hush-hush, finalize details with his agent and bring a limo. The brushed off alien contingent mutter a bit and go off to do as he asked, while Nesmith tries to corner Gwen who brushes him off with a "it was cute before I got to know you" line.
Nesmith eventually goes into the men's room and locks himself in a commode where he overhears two non-geeks who are making fun of the whole Galaxy Quest event and participants and calling the stars losers who haven't done anything in acting since the show, especially Nesmith who is bad-mouthed by his co-stars. Nesmith gets a wake up call and falls totally out of character for the rest of the day. He signs autographs with a "phone it in" mentality, doesn't look at the fans and when confronted by the real hosts of his next day gig turns on them, get's angry and tells them it's just a TV show.
Gwen overhears this and later that night talks with fellow co-star Dane about it on the telephone from their respective homes. She expresses concern about Nesmith and Dane just expresses contempt for the whole thing.
Nesmith, climbs into a bottle of booze on ice watching reruns of Galaxy Quest on TV and waking up when Mathesar and his fellow aliens come a knocking at Nesmiths' back door. Nesmith doesn't quite rise to the occasion and more like crawls around in his shirt and undies looking for shoes. Eventually they all get into a limo and head to the gig with Nesmith trying to make a play for the pretty alien girl Laliari (Missi Pyle) who can't speak because her translator device broke. Nesmith asks for a coke and promptly takes a nap as they explain the situation to him on the way up to the ship, which is a real Galaxy Quest space ship.
Once there, the unshaven, groggy and very unkempt Nesmith is taken to the bridge where he confronts the evil bad guy alien Sarris (Robin Sachs) on the Vid screen. Nesmith orders them to fire blue and fire red and fire other stuff. He says that should take care of Sarris and then he asks to be taken back home to Earth. Teb (Jed Rees) really appreciates that Nesmith has saved the people.
Nesmith is shown into a room where he is covered by goop that turns into a plastic shell and then he is shot out into space, landing at home by his swimming pool in a state of shock. He runs, as is, unshaven and unkempt, to the electronics store opening they have been contracted to do and tells his fellow actors about his real adventures in space with the "termites" as he calls them.
Everyone thinks he's drunk and crazy, even after Mathesar and his fellow aliens show up and want him and the crew to come up and negotiate with Sarris. Nesmith tries to convince his fellow actors to come along and help him. Instead they all give Nesmith the brush off and leave, climbing into the same van hostile and critical, except for Fred Kwan (Tony Shalhoub) who plays the Tech Sgt. Chen on the show and said they all should have taken the gig because who knows when Nesmith would offer them a gig again!
A gig? They all say, looking at each other and getting out of the van and meeting up with Laliari, whose image remains on Earth as Nesmith thought they might change their minds. Laliari orders up pods for all of them and in short order the acting crew of Galaxy Quest realize they are now on the real deal in outer space and that's a little more reality than they were planning on.
We see the aliens in their real form for the first time and they are a type of pastel octopus or squid who use a device to take human shape and speech.
Fred Kwan is the only one of the actors who is so out of it he's here now, probably due to too much Hollywood living, he is the only one blindly up for the adventure. Down deep inside he's as crazy as Nesmith only it didn't take him a first time to be that crazy. Gwen and Dane are the only sane ones who think it's totally insane to do this gig! Tommy's trying to deal with it all.
They get a tour of the facility, go aboard the ship where they get more of a tour and meet the crew and other "termites" as Nesmith calls them. They take their places on the bridge and take the ship out into space to meet with Sarris.
We learn that Mathesar and his people think Earth TV is serious documentary news and that shows like Galaxy Quest and Gilligan's Island are reality TV or new reports like the war in Iraq. Gwen and Dane take issue with Nesmith who is going along with this blindly because they think he's on an ego trip and maybe he is! He sure likes inspecting the troops.
The audience eventually discovers that Sarris had captured and tortured the last commander who told Sarris all about things on Galaxy Quest, including the coveted Omega 13 device, which Sarris wants to own at all costs. The "termites" don't know what the Omega 13 does, but badly-constructed, the device will disintegrate every matter atom in the universe. However, nobody knows where it is.
This sends the actors packing, except for Nesmith who still thinks he can handle Sarris. Well, he has to as Sarris has arrived and they must confront him. Sarris overhears Nesmith talking about his plans of attacking him with all their weapons because Gwen didn't follow Nesmith's order to cut the microphone, so he shoots first. The engine dies off, but synergy leads the starship to a landmine abandoned since an old war. When they leave the place, they manage to stop the starship.
The crew tries to explain that the Galaxy's adventures are a fiction, not historical documents, but the concept doesn't entirely resonate with the aliens: they have just started to understand the concept of "lie" and "deceit" as Sarris promises something and then does another thing. A new sphere of the energy source called beridium is found in a planet nearby, so they have to go and pick them. Arriving to the planet where the sphere is, it looks like the rocky Nevada desert, and they can breathe naturally. A kind of home appears. Some little monkeys are working on the mines. Guy is worried that he's the extra - the sixth crewmember- who hasn't got even a simple surname. One of the cute little monkeys is weak and sick, so the rest of the mining monkeys eat it alive.
The crew try to steal one of the beridium spheres by rolling it away. Hundreds of monkeys appear. The starship leaves with the sphere but without Nesmith, as Lazarus complains that Nesmith needs to be always the main protagonist. A large animal is about to eat Nesmith. The animal is tele-transported, and it dies painfully, becoming a ball of goo. The monkeys keep on shouting "Gorignak ", which is a monster made of rocks. Finally, Nesmith is tele-transported to the ship.
Back at the spaceship, Sarris has invaded the ship. Apart from that, Nesmith is told that there are no more people on planet Earth. Nesmith says that there is no use in torturing Mathesar, as he doesn't know what the Omega 13 device really is. He and Gwen show Sarris the "historical documents", who immediately understands the situation. He forces Nesmith to explain to Mathesar in simple words that a child can understand that they are not heroes, but only actors.
Sarris tells his thugs that they are going to make the starship explode with everybody inside. However, Nesmith and Lazarus pretend to fight, and they use the distraction to throw their guardians to the space void. Gwen reminds Nesmith that neither of them can activate a neutron engine. But Nesmith knows somebody who will know how to operate it. Brandon (Justin Long) is a nerdy geek who asked a lot of technical questions during one of the gigs. Gwen and Nesmith follow Brandon and his friend Kyle (Jeremy Howard)'s instructions. When asked what the Omega 13 device may do, Brandon says they think that it may allow to go back in time in 13 seconds. Hollister (Jonathan Feyer) discovers the rhythm of the smashing iron poles.
Lazarus and Quellek (Patrick Breen) open the door to bring oxygen to the Thermians. Laliari and Fred kiss like crazy. Finally, Nesmith reaches the blue button which will stop the explosion. Brandon can't help Nesmith and give him an alternate solution when the blue button doesn't stop the countdown because his mother, Katelyn (Kaitlin Cullum) forces him to take the rubbish out. Nesmith and Gwen hug, listening to the last seconds of the countdown. However, in the series, the countdown always used to stop a second before, and that's what happens.
Nesmith orders Tommy to drive the spaceship into the minefield. There, the magnetism of the ship attracts many mines. Nesmith's ship faces Sarris' ship, and goes down in the last second. The mines destroy Sarris' ship.
However, after going through a black hole, Tommy mentions an increase of energy from Sarris' ship. Sarris has entered Nesmith's ship. Pretending to be Fred, he wounds Nesmith and kills the rest of the crew. Nesmith shouts to Mathesar to turn on the Omega 13 device, so he arrives back in time - only 13 seconds, but time enough to hit Fred as soon as he enters the control room.
All the Thermians leave the spaceship, except for Laliari. They arrive at the convention they were supposed to attend - where thousands of geek fans are already disappointed they are not there - crash-landing with their spaceship thanks to Brandon and his friends. In the convention, each cast member leaves the ship half-suffocated and dizzy. Sarris also wakes up, but he is killed in a second, something which drives the fans wild.
CUE to the new season of Galaxy Quest - the cast includes a character called Laliari, performed by Jane Doe and who kisses Fred, and a new security lieutenant, interpreted by the extra actor Guy Fleegman.