The young girl is River, Simon's sister.  They are on the run from the government since he rescued her from a facility in which unknown tests were being performed.  Mal decides to unload the goods they have on Patience, a lady that shot him the last time they met.  During the deal, in which Patience tries to get something for nothing, Reavers arrive and the Serentiy must make a run for it.  Mal ends up keeping Simon, River and Book on a crew memebers.