Serenity employs an ingenious opening sequence to catch up viewers who are uninitiated in the Firefly 'verse, and then, in the words of River Tam, "I know. We're going for a ride." A thrilling ride.
As in his other work, Joss Whedon uses an intricate mix of genres, high drama, action, mystery, humor, and character driven plot to draw the viewer into his world, and then keep the audience on the edge of their seats for the next two hours. Serenity is scifi for the thinking person, where the knuckle-whitening action is there to serve the development of its characters, and the laugh out loud moments let you catch your breath almost in time to gasp in shock.
At the same time, the story works on the deeper level of analogy important to all great scifi, commenting on the human need for freedom, for family, for something to believe in, and the willingness of heros to sacrifice themselves for those ideals. Whedon weaves these threads throughout his story, and then trusts his audience to be able to enjoy the action and the deeper significance at the same time. Truly, a rare cinematic treat.
