For fans of “Firefly,” the short-lived Fox TV sci-fi series, “Serenity” is a must-see movie. For the rest of us, some explanation is in order.

Axed by the network in December 2002 after only 11 of the 14 completed episodes had aired, “Firefly” moved first to DVD, then to the Sci-Fi Channel and has become a cult phenomenon. So much so that writer/director Joss Wheadon of “Buffy the Vampire Slayer” and “Angel” fame, moved the whole works to the big screen.

That said, you don’t have to know anything about what happened on TV to see the movie. The opening sequence of the film effectively brings the entire audience up to speed about the Alliance that rules the galaxy 500 years into the future before introducing River Tam (Summer Glau), a 17-year-old telepath who is being mentally programmed by the Alliance as a human weapon.

She’s being sprung from captivity by her brother Simon (Sean Maher), who outwits security to smuggle her aboard Serenity, a rust-bucket Firefly transport vessel run by Capt. Malcolm “Mal” Reynolds (Nathan Fillion). Reynolds, a veteran from the losing side of the war the Alliance won, is now a rebellious smuggler/pirate who runs his ship under the galactic radar.

His crew is an appropriately wild bunch: Second-in-command is the muscled, tough Zoe (Gina Torres); her husband Wash (Alan Tudyk) is the pilot; Jayne (Adam Baldwin) is the crew’s gunner; and the sweet Kaylee (Jewel Staite), the ship’s mechanic.

To catch River, whose mind possesses information that could endanger the Alliance, the Operative (Chiwetel Ejiofor) is dispatched. A ruthless, amoral killer, the Operative is loyal only to the Alliance and willing to wipe out anything or anyone who gets in the way of his mission.

What follows is a wild ride through space that involves a payroll heist, rampaging Reavers (bands of vicious creatures who control the edge of the galaxy), battles between Alliance troops and the outmanned crew and at least a couple of romances.

If that all sounds like standard horse opera sci-fi stuff, it’s because that’s just what “Serenity” is. That it isn’t a lot different from many sci-fi movies past — essentially westerns set in the future — might stun some “Firefly” cult members. But that’s how it plays on film.

As is often the case when a television show makes a transition to the big screen, “Serenity” has its awkward passages. Wheadon is given too much to close-ups and interiors — the stock in trade of TV. And its pacing clearly derives from its original medium. You can almost feel where the commercials will go in the two-episode story line.

On the other hand, “Serenity” is mercifully bereft of the now overcomputer-generated imagery that rapidly becomes the heart of most contemporary science-fiction films. Instead, Wheadon keeps the focus on the interaction between the crew, its friends and enemies and manages to put some heart in the film.

Because the cast was taken from the TV show to the big screen, there are no big names to bring in the movie audience. But it’s clear that most of these folks have worked together in the past, and they have a natural camaraderie that works throughout the picture.

It’s hard to guess whether “Serenity” will find a film audience. If it does, Wheadon says he wants to make a trilogy out of the story. He should get more money for the second installment that will allow for a slicker film. His experience with “Serenity” should iron out the TV-to-movie transition that creates most of the film’s stumbling blocks.

Reach L. Kent Wolgamott at 473-7244 or kwolgamott@journalstar.com.