I missed watching the short-lived show Firefly when it was on TV. I caught a handful of episodes, but it seemed jumbled - later I found out Fox showed the episodes out of order. Also, they didn't air the pilot episode first, and that was important for the setting and plot.
But, via Netflix, I was able to queue and watch the series. It doesn't take long, since there were only 14 episodes!
The show was really good, a different spin on sci-fi. Some of the crew had fought on the losing side of an independence war, and were making their living at the edge of the control of the authorities (the Alliance), doing whatever work they could find. Not all of it was strictly legal - lots of smuggling and robbery - but you could describe the crew as honorable thieves.
The pilot started with Simon, booking off-planet passage for him and his cargo... and the nature of the cargo required discretion. He could pay, and not many more questions were asked, so captain Malcolm of the ship Serenity took them on, and they departed.
As it turns out, the Simon's cargo was his sister River, whom he just busted out of a fancy Alliance military facility. Since they were wanted fugutives, this compounded the difficulty of finding work for the crew. Throughout the shows, bits and pieces of various characters were revealed, including a little background on Simon and River. She was taken away and enrolled in some Alliance military training/experimentation program, and had some extensive psychological programming that left her a little unbalanced.
Anyway, the movie begins with more background - we see Simon busting River out, and the reason why the Alliance is chasing after them so intently: she is also psychic. While captive, the doctors showed her off to various officials, and she read their minds and learned state secrets. Back on Serenity, the crew leaves for a holdup - they plan to steal the payroll from an Alliance security contractor, because they need money for repairs and food. Midway through the holdup, Reavers attack and then the plot bounces around as they hide from the Alliance and the assassin they send, to seeking answers to River's condition on a mysterious planet.
I think somebody could enjoy this movie without seeing the TV series, but the series would help in understanding some of the characters they reunite with: Inara, Shepherd Book, and the Reavers, who were inadvertently created by Alliance terraformers.
Since this was written by Joss Whedon, there is a Buffy moment or two: River is a carefully programmed killing machine, and in one fight scene where she mows down Reavers (who are basically not human anymore), I was thinking of Buffy the Vampire Slayer versus demons. But hey, that isn't bad. :)