In case you haven't heard, the history of the TV show Firefly (upon which Serenity is based) harkens back to that of a little known television show from the 60's. You may have heard of it... it was called Star Trek.
Like the original Star Trek back in the 60's, Firefly gained a strong, hard-core following right from the outset. The problem was that the show (brainchild of Joss Whedon whose previous huge hit was Buffy the Vampire Slayer) aired on FOX network. It wasn't the fact that it was FOX so much as the fact that they showed episodes out of the intended order and instead of giving the show a chance cancelled it after only 11 episodes had aired.
Almost immediately the call went out among the small but dedicated fanbase to try to get another network to pick up the show. Unfortunately none of the networks wanted to carry the torch, but with brisk sales of the DVD version of the series, Universal decided to get a big screen version into production, and if word of mouth ends up carrying Serenity into big numbers, all those networks may end up feeling like CBS after they decided to go with Lost in Space instead of Star Trek.
I would actually recommend that you pick up the series on DVD since you can find it as ridiculously low as $30. Take a couple of days over the weekend to watch it and then go see the movie. You'll enjoy Serenity that much more.
Having made all the comparisons to Star Trek, the similarities end with the cancellation and popularity issues mentioned previously. Serenity is not about a grand mission to explore space, but is instead more of what would happen if a bunch of working class folks got a hold of a spaceship. Serenity is the name of the ship, a Firefly-class transport that Captain "Mal" Reynolds (Nathan Fillion) purchased as a near-junker sometime after a solar system-wide civil war has taken place, and his side (the Browncoats) has lost. He has a rag-tag crew (sorry for the cliche, but it fits) and they spend most of their time doing small-time jobs along the "outer rim", the outskirts of the solar system where the planets are more like the Old West than the gleaming spires of the inner, core planets. Included in the crew are a young doctor and a teenage girl who seems to be suffering from mental problems after having been a victim of experiments by the "Alliance", the government entity that won the war and is not as benign as the core planet citizens believe it to be.
The crew of Serenity has always known that the Alliance wants the girl (River, played by newcomer Summer Glau), but the government has stepped things up by putting a master assassin on the trail. The story takes us through the twists and turns of discovering what is so special about River and why the Alliance is after her.
That's all I'll say about the story, because the real strength of Serenity/Firefly comes from the characters. Joss Whedon (creator/writer/director) has crafted a great cast of complex characters. As a whole they function like a dysfunctional family that fights and bickers, with personalities that are not clear-cut... sometimes pulling them towards the easy way out while at other times pulling them towards doing the right thing. Mal is fiercely loyal to his crew, but he will not hesitate to put them in harms way if the stakes are high enough. He is strong, but not infallible and when in doubt, well he's still the captain and that's the end of it.
Among the rest of the crew we have Zoe (Gina Torres), his first officer who served with him during the war. Calm and reasonable, but you wouldn't want to be on the other side during a firefight with her. There's Wash (Alan Tudyk), her husband and pilot of Serenity, and with her toughness and his sense of humour they make an odd but endearing couple. We have Kaylee (Jewel Staite) the "Scotty" of the show, a wide-eyed (but not innocent) young lady to whom ship maintenance comes naturally. And then we come to my personal favorite Jayne, played by Adam Baldwin... He's the most "dirty" of the bunch as far as willingness to shoot someone or sell someone out for a reward, and he's the least intelligent of the group, but he's the most endearing character of the bunch. He's almost like a big kid that doesn't know any better and just follows his impulses. In the end, if he really considers you a friend he'll lay it all on the line for you.
There's also Ron Glass playing a small role as Shepherd Book, a minister whose mysterious backround contains more than meets the eye (his role was larger in the TV series). I don't recall seeing him in anything since his days on Barney Miller but I've always liked him as an actor and it was great to see him back on the screen.
If you're disappointed by the watered down, cardboard character sci-fi that Star Trek has become and by Star Wars' CGI-is-everything-and-characters-are-secondary style, you owe it to yourself to get the DVD set of Firefly and then go see Serenity. Just be warned that this isn't for the under-10 crowd as some of it (in both the TV show and the movie) is very intense and some of it pretty graphic.