Wow. Just … wow.
Spoilers would be criminal, so you won’t find them here. If you haven’t watched Firefly, Serenity will be a good, exciting, and comprehensible movie. If, however, you’ve seen the series, it becomes breathtaking.
So … what you do is, you buy or rent or talk a good friend into renting the whole series, you sit up one night and watch it from front to back (this can be done — I’ve done it), and then you go see the movie. Because it is too damned good a movie for you not to know how very, very, transcendentally good it is.