Serenity is a film written and directed by Joss Whedon. It is a sequel to the cult television series Firefly. It was released in August 2005.
From what I remember reading, it is set roughly 6 months after the events in the television series.
The plots seems to be focused primarily on the ongoing sub-plot in the show about River and the crew hiding her and her brother from the Alliance.
The film does fill in a few plot holes that were left by the TV series and does a good job telling the back story about how Simon got his sister out of the Alliance facility she was in.
It also tells the interesting story of the origin of the Reavers.
Definitely worth watching but I highly suggest you go out and buy the Firefly show on DVD and watch it first. Enjoy!