Over a year ago, I watched every episode of "Firefly" on the insistent recommendation of my brother, who is a huge fan of all things Joss Whedon. It's basically a western-style science fiction show, about a group of folks on a spaceship who do their best to avoid the law (they are smugglers and bank robbers) and the "Reevers"- a population of insane, bloodthirsty folks who eat their victims alive. That's over-simplifying it a lot, but it's the gist. I'm sure there are many, many geek-fan sites out there that would give you a much better synopsis than that.
I enjoyed it; I found it amusing, engaging and unique. It took me a while to really get into it, but I did enjoy it. But I wasn't in a hurry to watch the movie that the much-lamented, cancelled show culminated in: 2005's "Serenity". I can't explain why, I just wasn't. My brother kept encouraging me to watch it, though, and after loving what Whedon did with "The Avengers", I decided I'd better give it a look.
I did enjoy it, though it's not my favorite movie ever, and I'm not in a hurry to see it again. Mostly I liked Nathan Fillion (of course); he's as adorable as ever- manly, but just soft-hearted enough, funny but deadly serious when needed. The story was tight, the action believable and exciting, and the special effects surprisingly good for a not-huge-budget film. There is a small amount of sexuality; consider yourselves warned (if you're worried about that)- mild PG-13 level, I would say.
The sci-fi elements are really quite good- the story revolves around River, a girl who was taken by the government and experimented on because she's psychic. Skills and information were implanted in her brain, and the government wants her back. Not knowing the details, the crew of Serenity take her and her brother (who stole her from the govt. facility) aboard in the first episode of the series. The series does not revolve around this storyline, it is one of many, but the film "Serenity" is about this storyline and how it affects the crew, what the secrets are that the government will kill River to keep hidden, etc. It is very well written- intelligent and fascinating. In true Joss Whedon style, there is a lot of humor, exciting action and lots of surprises. Oh, and as my sister-in-law warned me (after my frustration at Whedon killing off Agent Colson in "The Avengers" this year) Whedon always kills someone you love. Which, as I've said before, lends a bit of authenticity to even the most outlandish story, simply because that's the most logical scenario. Someone will/would die.
If you haven't watched "Firefly", you can still watch "Serenity" and understand it, mostly, but I would recommend watching all of "Firefly"'s 14 episodes first, so you have a good feeling for the cast and the characters, as well as the setting. Just be prepared that the pilot ep. is extra long.