Galaxy Quest

The former cast of the once-popular television space-adventure series Galaxy Quest spend most of their days attending fan conventions and promotional stunts. Though Jason Nesmith (Allen), who played the commander of the NSEA Protector, thrives with the attention, the other cast membersâAlexander Dane (Rickman) as the ship's alien science officer, Fred Kwan (Shalhoub) as the chief engineer, Gwen DeMarco (Weaver) as the computer officer, and Tommy Webber (Mitchell) as a precocious child pilotâall resent these events.
During one event, Nesmith is approached by Mathesar (Colantoni) and others calling themselves "Thermians" and request his assistance, which he agrees to, thinking this is a planned and paying fan event. Later at that same convention, Nesmith becomes despondent after overhearing attendees speaking of him as a laughing stock by fans and his fellow actors, and he loses his temper with an avid fan, Brandon (Long). After Nesmith spends the night drinking heavily, the Thermians arrive to pick up a hungover Nesmith in the limo he had requested. Unaware that they are truly octopoidal aliens, using technology to appear human, the barely conscious Nesmith is oblivious to his limo being beamed aboard the Thermian's spaceship. Aboard their ship in deep space, Nesmith goes through the motions of commanding the ship and asks to be returned home. When they send him back to Earth via a transporter, Nesmith realizes that it is all real. He races to meet his cast, accidentally bumping into Brandon and misplacing a Thermian communicator Mathasar gave him with Brandon's fan-made replica. Nesmith eagerly relates his experience to the crew, who think he is drunk again. When another Thermian appears and request the entire crew's help, Nesmith manages to convince them, along with their handler Guy Fleegman (Rockwell), an actor who played a unnamed security officer on one episode before being killed off, to come along. They are all transported to a perfect reproduction of the NSEA Protector in deep space, and are shocked by the reality of the situation.
Mathesar begs the crew to command the Protector, as Nesmith's previous actions (namely, blowing up the opposing ship) have enraged Sarris (Sachs), a reptilian humanoid that seeks to wipe out the Thermians. While they were able to recreate the ship from the broadcast episodes, the Thermians have no idea how to pilot it. The crew hesitantly take the controls, and despite their ineptitude, the Thermians cheer them on. After the second encounter with Sarris' ship, they barely evade his attack by flying through a minefield, severely damaging the ship. The humans take a shuttle to a nearby planet to find a replacement beryllium sphere as a new power source. They manage to secure the sphere after a run-in with the hostile alien species on the planet. Once back aboard the Protector, they find that Sarris and his soldiers have captured the ship.
Sarris interrogates the humans, discovering they are only actors, and recognizes that the Thermians have no concept of fiction, believing the show to have been real. Sarris sets the Protector to self-destruct and departs, leaving a few sacrificial soldiers to guard the humans. Nesmith and Dane use a gambit from the show to engineer their escape, and then Nesmith orders his fellow cast members to help rescue the other Thermians, finish repairs to the Protector, and prepare to engage Sarris in combat. Nesmith and DeMarco then set off into the bowels of the ship to stop the self-destruct sequence, using help from Brandon and his group of friends via the swapped communication device. Along the way, they encounter Omega 13, a plot device introduced in the final episode but never used; Brandon notes it could either destroy all matter in the universe or rewind time by 13 seconds, "enough time to undo one mistake".
Having finally accepted their roles on the ship and gained confidence in themselves, Nesmith and his crew use the minefield as a weapon against Sarris' ship, destroying it. They prepare to head to Earth when Sarris, who has transported over at the last moment, starts killing the crew. A desperate Nesmith activates the Omega 13, which reverses time far enough for him to knock out Sarris. They near a wormhole to return the humans home via the command module, and Nesmith assures Mathesar he has the ability to command the Protector along with the other Thermians. The humans, along with Laliari (Pyle), a Thermian that has fallen in love with Kwan, return home. The command module crashes into Earth near a fan convention and comes to a stop after crashing through one wall, which the audience takes as part of the show. As the crew exits the module, Sarris wakes up and tries to fire on them, but Nesmith reacts faster, and disintegrates Sarris with a phaser-like weapon. The crowd erupts into cheers. Some time later, Galaxy Quest is revived as a new series, starring the same cast along with Fleegman and Laliari.

Question: What do the Thermians have no concept of?
Answer: Fiction