Yeah, you have to see the firefly series before you watch serenity. Serenity was an attempt to reignite the firefly fan base and get the series back on the air.
Firefly, for those who didnt see it, was a cult sci-fi/western composite. Basically the 'evil empire' controls the galaxy and after a brief insurgency attempt some of the insurgents do 'jobs' with their crappy little space ship out at the edge of the universe where the evil empires grasp is limited. Due to the distance from the home worlds, the technology is limited, mostly people with a few horses to plow the land, regular guns, etc. Most of the on-world action looks like a western; off world stuff looks like sci fi.
The problem with the series is that the network decided that the 2 hour pilot, where all the characters were introduced and what the heck was going on was deemed "not exciting enough", so they started running the series with a later episode and didnt run the pilot until mid season, after they had already decided to cancel the series. The series experienced an uplift when sci-fi channel reran the whole thing in the correct order but without a big broadcast audience, it didnt...umm...fly.
Really disappointing considering that it was at least a little innovative and how many really, really bad rubber stamp shows persist into their 4th, 5th and 6th seasons...
No more though; the shows creator has thrown in the towel and given up.