It's not often that I reward a film with 10 stars, but Serenity is one of those that just works in every single way. I might be a little biased because I also think that Firefly, the series on which the film is based, is the best series ever made, so keep that in mind as you read the rest of this review.

The story pretty much follows the happenings in the series and adds a little history to the mix. We see how River Tam (Summer Glau) is rescued from a government facility where she is experimented on for her mental abilities (including telepathy). Her brother Simon (Sean Maher) manages to get her out of the facility and, as it was shown in the series, becomes the medic on a ship called Serenity. In response to the rescue operation, the Alliance (A collaboration between several parliaments from different planets throughout the universe.) sends out an assassin to capture River and Simon.
In the mean time, the crew of the Serenity, lead by captain Malcolm 'Mal' Reynolds (Nathan Filion) is planning a heist on a small planet. Together with his second in command Zoe (Gina Torres), hired gun Jayne (Adam Baldwin) and Summer they plan on stealing some money from a local bank when their plan is rudely interrupted by Reavers; a race of humans gone mad when they saw the edge of space, and who rape, murder and pillage their way through life.
They manage to escape with the booty and make their way to the rendezvous where they meet up with the men who gave them their assignment. At this bar, a subliminal message triggers something in Summer's brain, making her beat up everyone and everything in the near vicinity. Thanks to this stir-up, the Alliance's assassin is able to track down the Serenity and Summer with it. You're going top have to find out about the rest for yourself.

The things that make Serenity such a great film are almost too many to count. Even though the series was a somewhat better, the film does not disappoint the fans of Firefly. The characters are as awesome as they were in the series and we get to know even more about their backgrounds, the characters that are introduced, such as the assassin (Chiwetel Ejiofor) and Mr. Universe (David Krumholtz), fit inside the story perfectly and the entire feel of the movie is just as it should be: laid-back with just the right amount of fuck-yeah.
Another thing that adds to the mix is the realism. Even though it is a sci-fi that takes place about 50 years into the future, it looks like it could actually happen. The planets and cities look a lot like what we have now or what we used to have; little towns straight out of old westerns, harbours that seem to have taken a hint from the ones in Titanic, etcetera. Of course there's also the huge futuristic cities on the Central Planets where the Alliance rules, and there's lasers and spaceships and what not, but still, it looks like it could happen.
The third thing is something that I noticed right away and loved so much it's creepy. It's simply the lack of sound in space. Apart from the soundtrack (which is, by the way, beautifully done) and the dialogues, you hear absolutely nothing when there's shots filmed in space. Explosions, crashes, engines firing up, everything is absolutely silent. I love that. There's tons of sci-fi that has sound in space, even Star Wars is guilty of this, but Joss Whedon (The guy that also brought us Buffy and Angel) said 'Fuck that noise' and went for realism 'till the end.
There's only one thing that's a bit of a let down to me, but that's mostly because I am a huge fan of the series. It seems that, while making this film, the crew didn't pay as much attention to detail as they did in the series. For the series, the Chinese used was always carefully translated, give or take a different symbol for the same word here or there. In the film, a lot of Chinese is nonsense. Of course, this doesn't do anything to the feel of the movie. it's just some tiny details the hard-core fans might notice.

All in all, Serenity is definitely worth your time, but personally, I would recommend watching the series first. It's just 14 episodes and it adds so much to the story of the film, you'd be crazy to miss out on that.