Joss Whedon’s rowdy space western represents a odd quirk in movie franchises: a feature follow-up to a flop television series. Perhaps that was inevitable after Hollywood had trashed its way through so many of the old TV shows that were hits, but it’s still a woeful prospect. It’s horrifying to imagine the thousands of justifiably deceased programs the studios might now unearth for similar treatment. To use a rather unpleasant analogy, it seems rather like trying to feed on carrion that even the vultures have shunned. (If the trend should catch on, may I suggest “My Mother the Car” for big-screen resuscitation? In a nod of the hat to this movie, that feature could be titled “Obscenity.”) In any event, “Firefly” had such a brief run on Fox that all the thirteen episodes made weren’t even broadcast by the “fourth network” (the SciFi Channel is now running all of them). But it had a loyal (if small) fan base–enough, apparently, to justify green-lighting this big-screen exhumation. Devotees of “Firefly” will probably enjoy “Serenity.” But most everybody else will find it a small-screen sort of product, out of place among Hollywood’s mega-budget special-effects extravaganzas. (Not that they’re much better, despite the money behind them.) And the fact that it’s shot in widescreen makes no difference: letter-boxing is all the rage among TV series nowadays.

The basic premise of the original show was obviously patterned on “Star Trek” and the initial episode of “Star Wars,” with the titular ship, a rust-buckety old vessel, coasting from weekly adventure to adventure, not as an emissary of some federal government but as a free-wheeling relic of the day before the planetary outposts of old earth had fallen under the thumb of a repressive regime called The Alliance. (Happily, the movie gives us all the back story pretty quickly up-front.) The crew is a motley assortment of troublemakers, smart-alecks and serious types who trade the usual juvenile barbs and attempt halting romantic advances while trying to avoid capture by The Alliance and, generally, fighting the system with piratical elan. Heading it is the wise-cracking Captain Malcolm “Mal” Reynolds (Nathan Fillion), who might best be described as the younger cousin of Han Solo. He’s aided by ace pilot Wash (Alan Tudyk); his buccaneering wife Zoe (Gina Torres); a sweet young girl named Kaylee (Jewel Staite), who’s a prettier equivalent of James Doohan’s Scotty; and the abrasive rogue Jayne (Adam Baldwin). Also on hand is a straight-laced doctor, Simon Tam (Sean Maher).

The plot of this episode has to do with the doc’s sister River (Summer Glau), a telepathic teen whom Simon has rescued from an Alliance laboratory. Her presence on the Serenity leads to its pursuit by a cold-hearted, implacable Alliance agent (Chiwetel Ejiofor)–so off-the-books that he doesn’t even have a name!–and to what seems an uncountable number of chases (on land and in space), fights (most involving advanced martial arts), and climaxes real and imagined. The plot encompassing all of them has something to do with the origin of an unfortunate race of murderous plunderers called The Reavers, who might be described as Klingons gone really bad–a secret to which River apparently holds the key, and which would endanger the Alliance if revealed, thus explaining the government’s desperate effort to reclaim her. Along the way several other characters are introduced: a leader called Shepherd (Ron Glass) at an isolated outpost who shelters our intrepid heroes; a high-class hooker (I think) named Inara (Morena Baccarin), who’s apparently an old flame of the captain’s; and a self-employed broadcaster called, for reasons of the reach of his signal rather than his physique, Mr. Universe (David Krumholtz).

To initiates witnessing the goings-on, “Serenity” will be like attending a family reunion with relatives you like, and they’ll enjoy spending more time with these characters and have no trouble understanding the ins and outs of the plot. Those unfamiliar with the show will probably have more difficulty falling in love with the people aboard the ship and coming to terms with the futuristic setting and the somewhat bumpy scenario that writer-director Whedon has cobbled together from bits and pieces of old movies (mostly westerns), TV shows, comics and boys’ adventure magazines. But on its own “Serenity” comes across as neither enthralling nor terrible. The acting, apart from Ejiofor–who fashions an elegantly cool villain–is basically pedestrian, and the effects are at the lower end of today’s sci-fi spectrum. But the script–apart from those inevitably mawkish moments when someone decides to talk seriously about life and duty, or remembers a friend who’s no longer among the living–is reasonably lively, with plenty of zingers and oddly-phrased locutions that are hardly examples of wit but have a certain schoolyard punch; and for the most part director Whedon keeps the action moving along fairly well until the last act, when he not only stretches out a point-counterpoint fistfight and gun battle too long but then adds a sappy coda we might have done without. And though the overall design (by Barry Chusid) and camerawork (by Jack Green) lacks any sense of majesty, they’re certainly true to the small-screen origin of the project. Even David Newman’s score seems stock.

And ultimately that’s the underlying problem with “Serenity.” It comes across like a perfectly acceptable elongated episode of a television series projected onto a screen several sizes too large for it. On DVD or cable, it would be a hoot. At the multiplex, though, it’s more pretender than real contender.