Serenity achieves something that I wasn't sure was possible: it should end up appealing to fans of the show Firefly as much as it does to people who didn't even know that it was derived from an unfortunately short-lived TV show. The early exposition is kept to a minimum and even provides a little bit of background that will keep the fans happy, and, with just that tiny bit of fuss, the viewer is launched into Joss Whedon's Western-tinted science fiction world.
The plot is fairly typical: fugitives from a secretive, scary government are sheltered by a group of (mostly) good-hearted outlaws. The group is hunted across the galaxy and share wild adventures. But that isn't the half of it.
The script is smart and funny and quick-witted, the characters are sharply drawn, and it manages a few emotional tugs along the way (tugs that will be enhanced for fans of the show). From the very beginning, there are surprises and the action moves with impressive intensity. This movie is, almost literally, one that will have the crowd sitting on the edge of the seat, eyes wide, and wondering what's going to happen next. This is good stuff.
For as action packed as it is--and the action makes the thing move fast--it never seems stupid or gratuitous. In fact, some of the more casual and brutal violence makes the most sense in the context of the story. It isn't overly bloody, it isn't sickening, and it isn't over-the-top; it's a movie where the violence simply makes sense.
The characters--from Jayne with his hilarious, mercenary self-interest to River with her mysterious past and to the Captain, Mal, with his conflicted worldview--all bring something unique and interesting to the show. They never feel like stock characters that would fill out any other script or story--they are a product of Whedon's well-conceived universe.