River is being, through some type of psychosis, conditioned into a type of assassin and her brother, the doctor, continually hides information from the crew as to what her importance is and why, exactly, the Alliance is chasing the crew to find her.
It is only hinted at, early in the film, that she may have been privy to some sensitive, top-secret, governmental information. But here, the plot becomes convoluted. It is never really told why the Alliance gassed the planet of Miranda or why River, except the given reason that she just "picked it up" during her treatment to become an assassin, is interested in this planet. The plot rests on no real evidential or emotional foundation. Any information given as to why all of this is happening is skimmed over at best - nothing is really given the amount of screen time it deserves in regards to plot explanation.
I never really felt connected with any of the characters - I never felt invested in the problems at hand. Why should I care the planet was gassed, all its inhabitants killed, when River, the person most connected, although only through association, with the event didn't seem to really care? The only time I felt truly connected was when Malcolm, the captain, fought within himself when confronted about leaving a settler behind to become a victim of the Reavers early in the film.
Overall, Serenity is a decent movie worth a rental. It has some serious plot holes and is far too spastic in plot and acting leading the viewer to never really know the intentions of the characters or the storytelling.