Serenity is the 2005 follow-up to the short-lived 2002 Fox television series Firefly. The movie reunited show creator Joss Whedon with Nathan Fillion (Mal Reynolds), Summer Glau (River) and the rest of the original cast. Sarah Paulson joined the cast for the film. The film is classified as a space western. Serenity is set in the 26th century, when Earth's population is spread out among several planets with various levels of technological and economic success. Mal and other members of the Serenity crew fought in a civil war against the Alliance, which is made up of the wealthier planets, and the Alliance won the war.
On board the Serenity is River, whom the Alliance performed experiments on, giving her skills like being an incredible shot and martial arts skills. They also damaged her psyche. Mal, captain of the Serenity, is using her skills as they perform their less-than-legal jobs across the universe. During one of these jobs, River attacks without warning and then whispers the word "Miranda."
A hacker contact tells the Serenity crew that the Alliance is broadcasting a message to River activating River's outbursts. They conclude "Miranda" must be connected to the message and discover there was an Alliance colony named Miranda.
On their way to the colony, Serenity runs into Reavers, a group of humans who live on the outskirts of space and are somewhat akin to space zombies.The Alliance government denies their existence. The Miranda colony is deserted, except for dead bodies. A log is found explaining that Miranda was the site of human experiments by the Alliance designed to suppress aggression. Instead it made most of the population slowly lose the will to live, and they died of dehydration. The remainder had a different reaction to the chemicals and became the Reavers.
The film ends with a battle between Serenity, the Reavers, and the Alliance. The Alliance is forced to officially acknowledge the existence of the Reavers. The Serenity crew suffers heavy losses.