Serenity is a 2005 American space Western film written and directed by Joss Whedon in his feature directorial debut. The film is a continuation of Whedon’s short-lived 2002 Fox television series Firefly and stars the same cast, taking place after the events of the final episode. Set in 2517, Serenity is the story of the crew of Serenity, a “Firefly-class” spaceship. The captain and first mate are veterans of the Unification War, having fought on the losing Independent side against the Alliance. Their lives of smuggling and cargo-running are interrupted by a psychic passenger who harbors a dangerous secret.

In the 26th century, humanity has colonized a new solar system. The central planets formed the Alliance and won a war against the outer planet Independents who resisted joining the Alliance. Child genius River Tam is conditioned by Alliance scientists into becoming a psychic and an assassin but is rescued as a teenager by her brother Dr. Simon Tam. During her training, River inadvertently read the minds of several top government officials and learned their secrets. Consequently, an Alliance agent known only as the Operative is tasked with recapturing her.

The siblings have found refuge aboard the transport spaceship Serenity with Captain Malcolm “Mal” Reynolds, first mate Zoe Washburne, her husband, pilot Hoban “Wash” Washburne, mercenary Jayne Cobb, and mechanic Kaylee Frye. Despite Simon’s objections, Mal brings River on a bank robbery. River warns them that savage and cannibalistic Reavers are coming. They escape, but Simon decides he and River will leave Serenity at the next port. Once there, however, a subliminal message in a television commercial causes River to attack numerous bar patrons, and Mal takes the siblings back aboard the ship. The crew contacts reclusive hacker Mr. Universe, who discovers the message designed to trigger River’s mental conditioning. He notes River whispered “Miranda” before attacking and warns that someone else has viewed the footage.

Mal receives an invitation from courtesan Inara Serra. Realising it is a trap, Mal goes to confront the Operative who promises to let him go free if he hands over River. Mal barely escapes. Miranda is discovered to be a planet located beyond a region of space swarming with Reavers. The crew flies to the planet Haven but find it devastated and their old friend Shepherd Book mortally wounded. The Operative promises to kill anyone who assists them until he gets River.

Mal has the crew disguise Serenity as a Reaver ship and they travel to Miranda undetected. They find its 30 million colonists dead, and a recording that explains an experimental chemical to suppress aggression had been added into Miranda’s atmosphere. The population became so docile they stopped performing all activities of daily living and placidly died. A small proportion of them had the opposite reaction and became insanely aggressive and violent. The Alliance had accidentally created the Reavers and this was the secret in River’s subconscious.