The crew of the classic 1980s space-adventure TV series, Galaxy Quest, now spends their time at fan conventions and promotional events. While the show's arrogant former star, Jason Nesmith, who played Commander Peter Quincy Taggart, revels in the attention, his co-stars—Gwen, Alexander (the ship's science officer), Fred (the chief engineer), and Tommy (the young pilot)—harbor resentment towards him due to his inflated ego and how he undermines their contributions.
During a convention, Jason encounters a group called the Thermians, led by Mathesar (Enrico Colantoni) and his aide Teb (Jed Rees). Mistakenly believing they want him for promotional work, Jason is shocked to learn that they require his assistance for a real interstellar crisis. After a night of heavy partying, he awakens to find himself aboard a genuine replica of the NSEA Protector, ready to face their enemy, the sinister warlord Roth'h'Har Sarris (Robin Sachs). Unbeknownst to him, Sarris is after the "Omega 13," a powerful weapon. Though he blunders through his role, Jason manages to fend off Sarris temporarily.
When Jason returns to Earth, he realizes the adventure was genuine after he and a fan named Brandon exchange communicators. Desperate to prove his reality-altering experience to his fellow cast members, he is initially dismissed. However, when the Thermian Laliari (Missi Pyle) appears again asking for help, the cast, thinking this is a performance gig, grudgingly joins him, including Guy, who played a red-shirt character.
As they board the Protector, the cast learns that the naïve Thermians have based their entire society on the show's exploits, believing them to be factual history. Mathesar reveals how their previous commander was killed by Sarris. Sarris returns with a vengeance, capturing the ship and threatening the lives of the crew with his demands.
In a daring escape, they head to a nearby planet to salvage a damaged power source, only to be attacked by a horde of alien creatures. Jason courageously sacrifices his safety for the others, but luckily, Fred uses the ship's technology to rescue him just in time. Back on the ship, Sarris lays siege, driven by revenge and a thirst for the coveted Omega 13.
As the self-destruct mechanism is activated, the cast must work together, along with guidance from fans on Earth led by Brandon, to abort the explosion and vanquish Sarris's forces. Amid the chaos, they regain control of the Protector and formulate a plan to turn the tables on Sarris, utilizing the magnetic minefield to their advantage.
In a final showdown, Jason ingeniously activates the Omega 13, allowing them to reverse time by 13 seconds, giving him a critical window to dismantle Sarris's plans once and for all. The chaotic finale brings them crashing back to Earth, where they land right in the middle of a Galaxy Quest fan convention, much to the surprise and delight of their loyal fans—including Brandon and his friends.
The exuberant crowd mistakes the climactic battle as an elaborate performance, while Jason heroically obliterates Sarris. With the cast back in the limelight, they bask in the admiration of their supporters, and later, they find themselves reunited for a sequel series, Galaxy Quest: The Journey Continues, reprising their beloved roles alongside new characters.
