Firefly was a sci-fi TV series by Angel and Buffy: the vampire slayer's creator Joss Weadon. It lasted less then a regular season (15 episodes) but was brought back a few years later as a serenity and as a number of comic books mini-series.
The stories revolve around a group of smugglers living onboard a firefly-class starship (hence the series' title) named Serenity (hence the movie's title). Its captain and first officer were soldiers in the Independents army (nicknamed "Browncoats") who fought, and lost to, the Alliance during the Unification War. The Alliance, which is made up of the central planets of their solar system (the humans fled the "Earth-that-was" to a new system they refer as "the 'Verse") now control (more or less) all the human-inhabited worlds in a fashion that range from enlighten autocracy to downright dictatorial.