For questions about "Serenity", a movie released in 2005, starring the crew of the Firefly TV series.
Serenity is set after the events of the firefly television series, an American space western television series created by writer and director Joss Whedon.
Serenity is the name of the Firefly-class ship that the nine main characters live on. The movie focuses on River Tam, providing her back story prior to being rescued by her brother Simon, after which the two board Serenity as passengers (originally shown in the Firefly television series).
A government agent called the Operative is sent to kill River, as she has been visited by high Alliance politicians and may have learned certain top secrets due to her psychic abilities. The movie features their fleeing from the Operative as well as filling in pre-existing gaps in the Firefly universe.