Gene Roddenberry famously pitched his original Star Trek (1966) as "Gunsmoke set in outer space." Joss Whedon actualizes that pitch with Firefly -- right down to the long-barrel pistols, cattle rustlers, and hyup-flavored rustic dialogue. After Earth becomes uninhabitable, a slew of Eastern Rim planets are terraformed through Yankee ingenuity. Eventually, civil war erupts between the Alliance (a kind of corporate government, formed by merged superpower companies from China and the US) and the "Browncoats," consisting of a scrappy clutch of small outer planets and moons declaring their independence; the Browncoats fight valiantly but are ultimately doomed. The truncated series takes place a few years after the end of this Unification War.
Mal Reynolds, former Browncoat, now captains the Serenity, a Firefly-class cargo/transport ship. He retains his ideals (the ship is named for the bloody, deciding Battle of Serenity Valley), buried as they are in a life of smuggling operations, the occasional Robin Hood-like gesture, avoiding the Alliance whenever possible. This quiet, personal rebellion is endangered when he takes onboard Doctor Simon Tam and his sister, River. Turns out River suffers from a lab-created psychosis, courtesy of the Alliance, who now want her back -- discovering the secrets of her strange powers and knowledge, and the quest to keep her hidden and safe until she can be brought back to normal, are the undercurrent story threads for Firefly.
But the show is truly buoyed by its characters, all of whom are cookie-cut in the shape of stereotypes, but crumble to subvert those very stereotypes. Serenity's crew consists of: loyal first mate Zoe, who fought beside Mal in the war and has never left his side. Zoe is married to wise-cracking Wash, pilot and resident peacemaker. Jayne Cobb is the proverbial numbskull thug-for-hire, always carrying too much ammo, and ready to rat out the Tams for the reward money. Kaylee is the ever-beatific mechanic, radiant despite ever-present swatches of engine grease on her cheeks (and not-so-secretly in love with Simon). Shepherd Book is a mysterious, dogma-free holy man -- never actually delivers a sermon, still functions as moral compass. Inara is Serenity's associate "Companion," a cross between a Geisha and a high-class call girl; Mal disapproves of her profession but keeps her onboard out of necessity (she's often their cover for landing clearance on Alliance planets) ... and out of underlying romantic friction.
After the fashion of the characters, several episodes make use of stock plots, only to transcend them. There's a train robbery; an outlaw township whose citizenry worships Jayne as a folk-hero; a femme fatale who tricks Mal into marrying her so she can pull off a heist; even a brothel-set recasting of Seven Samurai (1954). The characters, continually cracking wise, move through these scenarios with a kind of "Oh, this old story" attitude that refreshes the entire enterprise. (And there's blessedly little "reverse the polarity of the neutron flow" gibberish, even in "Out of Gas," the one episode revolving around the technical operation -- or lack thereof -- of Serenity.) The milieu is so complete and enchanting that the lack of any scaly or slimy aliens is hardly noticeable. This is a human 'verse -- these characters and the worlds they inhabit feel real and knowable, and you want to keep knowing them...
see also: Serenity (2005)