The show takes its name from the "Firefly-class" spaceship, Serenity, that the central characters call home. It resembles a firefly in general arrangement, and the tail section, analogous to a bioluminescent insectoid abdomen, lights up during acceleration. The ship was named after the Battle of Serenity Valley, where Mal and Zoe were on the losing side. It is revealed in "Bushwhacked" that the Battle of Serenity Valley is widely considered the loss which sealed the fate of the Independents.

Throughout the series, the Alliance is shown to govern the star system through an organization of "core" planets, following its success in forcibly unifying all the colonies under a single government. DVD commentary suggests that the Alliance is composed of two primary "core" systems, one predominantly Western in culture, the other pan-Asian, justifying the mixed linguistic and visual themes of the series. The central planets are firmly under Alliance control, but the outlying planets and moons resemble the 19th-century American West, with little governmental authority. Settlers and refugees on the outlying worlds have relative freedom from the central government, but lack the amenities of the high-tech civilization that exists on the inner worlds. In addition, the outlying areas of space ("the black") are inhabited by the Reavers, a cannibalistic group of nomadic humans that have become savage and animalistic.

The captain of the crew of Serenity is Malcolm "Mal" Reynolds (Nathan Fillion) and the episode "Serenity" establishes that the captain and his first mate Zoe Washburne (Gina Torres) are veteran "Browncoats" of the Unification War, a failed attempt by the outlying worlds to resist the Alliance's assertion of control. A later episode, titled "Out of Gas", reveals that Mal bought the spaceship Serenity to continue living beyond Alliance control. Much of the crew's work consists of cargo runs or smuggling. One of the main story arcs is that of River Tam (Summer Glau) and her brother Simon (Sean Maher). River is a child prodigy, whose brain was subjected to experiments at the hands of Alliance scientists at a secret government institution. As a result, she displays schizophrenia and often hears voices. It is later revealed that she is a "reader", one who possesses telepathic abilities. Simon gave up a career as a highly successful trauma surgeon to rescue her from the Alliance, and as a result of this rescue they are both wanted fugitives. In the original pilot "Serenity", Simon joins the crew as a paying passenger with River smuggled on board as cargo. As Whedon states in an episodic DVD commentary, every show he does is about creating family. By the last episode, "Objects in Space", the fractured character of River has finally become whole, partly because the others decided to accept her into their "family" on the ship.

imdb:tt0303461