Galaxy Quest (1999)

Eighteen years after their sci-fi adventure show "Galaxy Quest" was canceled, actors Jason Nesmith, Gwen DeMarco, Alexander Dane, Tommy Webber, and Fred Kwan are making appearances at sci-fi conventions and store openings in costume and character. They're wallowing in despair and at each other's throats until aliens known as Thermians arrive and, having mistaken the show for fact and consequently modeling their entire culture around it, take them into space to save them from the genocidal General Sarris and his armada.