You are actors on a space western about revolutionaries who lost the war.
But these aliens who showed up just aren't understanding the "actor" part. They want you to help them fight in their own civil war, and they won't take "no, thanks anyway" as an answer-- or at least, it doesn't appear that way, since they just abducted you and you're on your way to their homeworld.
While the setting is intended to be Firefly-esque, you do not have to play the same actors playing the same characters-- your pilot can be someone other than Alan Tudyk, playing someone other than Wash.
Players are not expected to be scholars of either Firefly or Galaxy Quest, but it would be helpful if they were aware of the general plotlines and tones of both.