“Serenity” may not be the first word that comes to mind when you envision several desperate men and women fighting for their lives against a horde of ax-wielding, half-human cannibals, but Josh Whedon has always been a bit of an ironic fellow. His rousing new space opera, “Serenity,” is the best interstellar epic to come along in many moons.

Mr. Whedon is best known as the creator of the television series “Buffy the Vampire Slayer” and its spin-off, “Angel.” Less successful was his sci-fi series “Firefly,” which was abruptly canceled by Fox partway through its first season. But, as any fan of Mr. Whedon’s vampire sagas can attest, he has a penchant for bringing things back from the dead. When he felt his script for the original, early-1990s film version of “Buffy the Vampire Slayer” was brutalized by the studio, he reinvented the concept as a television show. This time he’s done the reverse: Bolstered by strong DVD sales of “Firefly,” he’s resurrected the series as a motion picture, featuring the original cast and following through on his intended storyline. Who knows? If it’s successful – and it deserves to be – maybe he can get another series out of it.

“Serenity” opens with a bit of hokey exposition set in a children’s classroom, in which it’s explained that after Earth became too crowded, humankind departed for another solar system full of planets that could be terra-formed into inhabitability. This system is ruled, to varying degrees, by the domineering Alliance government, which recently won a war against a number of independent planets that had the gall to want to stay that way.

No sooner does this history lesson start to feel like, well, a history lesson, than it is violently subverted – the pleasant lectures are part of some gruesome brainwashing – and then subverted again, in a wicked feat of cinematic showmanship. In the process of this narrative triple gainer, we meet River Tam (Summer Glau), a uniquely gifted young woman upon whom the government has conducted hideous tests, and her surgeon brother Simon (Sean Maher), who engineered her escape from the lab. We also encounter the movie’s chief antagonist, a nameless and impossibly cool Alliance operative (Chiwetel Ejiofor) whose job it is to retrieve River.

Next we flash to Summer and Simon’s post-escape home, the spaceship Serenity, where a witty Steadicam stroll introduces the craft’s motley crew, most notably Captain “Mal” Reynolds (Nathan Fillion), a veteran of the war against the Alliance who has since become a smuggler and thief, though one with a stern sense of duty. The ship is in the process of a somewhat bumpy crash landing (“We may experience some slight turbulence – and then explode,” Mal warns over the intercom), after which captain and crew undertake a bit of good-natured robbery. The heist is interrupted, however, by the arrival of a pack of Reavers – degenerate, self-mutilating barbarians who advocate rape, murder, cannibalism, and, if the sooty exhaust from their ships is any indication, the abolition of the catalytic converter.

Throughout the rest of the film, the crew of Serenity are caught between lethal extremes of chaos (the Reavers) and order (the Alliance), a pairing that ultimately turns out not to be a coincidence. Our heroes discover what it is inside River’s head that the Alliance is so eager to keep from getting out, and follow that secret to a dead, forgotten planet that will shed light not only on the Alliance but on the Reavers as well. Along the way, there are several firefights, another crash landing, and two ass-whuppings by River that put Buffy’s best work to shame. The movie concludes with a quasi-libertarian moral about the danger of social engineering and the imperfectability of man that is more interesting – and vastly more coherent – than any of the windy philosophizing George Lucas has given us of late.

“Serenity” may be the closest thing to the original “Star Wars” since “The Empire Strikes Back” – an audacious space fantasy full of action and imagination, alternately hilarious and harrowing. As with the original “Star Wars,” one has the sense that there is much more to this universe than Mr. Whedon has been able to fit on screen, that this story is but part of a vaster history of individuals and empires still locked in his head.

But if this is a strength of the film, it is a weakness, too. It feels as though a full season of “Firefly” episodes has been boiled down into two hours. And there are simply too many characters – nine holdovers from the series, plus a few co-stars – to develop for viewers not already familiar with the show. It doesn’t help that the only cast member who exudes big-screen charisma is Mr. Ejiofor (“Dirty Pretty Things”), who very nearly steals the film as the Alliance’s over-civilized assassin. Visually, too, the movie’s television roots show a bit, with effects and cinematography that are exuberant but hardly cutting edge.

Like “Buffy” and “Angel,” “Serenity” has an unexpected narrative ruthlessness, with more than one cast member not making it to the final reel. Mr. Whedon’s is a dark universe, but still one well deserving of a visit. Just don’t get too attached to anyone you meet there.