As recent history has shown, making a movie based on a classic TV series is almost always a bad idea. But what about a movie based on a cancelled TV series? That's the unique position Universal Pictures finds itself in with Serenity, a feature-film version of the short-lived sci-fi series "Firefly." Although the low-rated show was banished from the airwaves after only 11 episodes, creator Joss Whedon (who was also responsible for two of the best TV shows of the past decade, "Buffy the Vampire Slayer" and "Angel") refused to let his baby die and after shopping the project around to various studios, he found a willing partner in Universal. Now Firefly lives again as a $50 million feature film, complete with the original cast and Whedon in the director's chair. The show's devoted fan base has been breathless with anticipation since Serenity was announced and it's unlikely that the finished product will disappoint them. What's less clear is whether a wider audience will respond to the film, which, while accessible to newcomers, demands a certain level of familiarity with the series.

Truth be told, "Firefly" was always something of a mixed bag. An ambitious attempt to marry the western and sci-fi genres with Whedon's trademark witticisms along for the ride, the show followed the outlaw crew of the spaceship Serenity as they pulled off the odd caper and tried to avoid the long arm of the law. Early on, "Firefly" struggled to find the right tone and while it improved as it continued, the series still suffered from uneven performances and inconsistent scripts.

Nevertheless, there was a great deal of potential in the premise and, in many ways, the big screen turns out to be a more suitable home for Serenity (née "Firefly") anyway. For one thing, it allows Whedon to bring more visual detail to his universe than he could on television. The titular spaceship feels like a lived-in vessel rather than just a set on a soundstage. The writer-director has also reined in some of the more overt western flourishes, which were generally the weakest part of the series. Instead of having horses wander through a scene, the movie channels the genre primarily through cinematography and camera framing. (The DP is Jack Green, who also lensed Unforgiven.) The film unfolds in a series of beautifully composed widescreen images that reference everyone from John Ford to Sergio Leone. Whedon also has the budget to stage some exciting and well-choreographed action sequences, including a climactic shootout that is reminiscent of The Wild Bunch. If nothing else, Serenity proves that he has what it takes to direct a big-budget feature film, which bodes well for his next project, the long-in-the-works movie version of Wonder Woman.

Where Serenity falters is in the story that Whedon has chosen to tell. Rather than use this opportunity to plunge viewers into an all-new adventure, the overarching narrative is driven by a mystery left over from the series. That mystery's name is River Tam (Summer Glau), a seemingly ordinary girl who was subjected to horrible mind-control experiments at the behest of the Alliance, the regime that runs this corner of the universe. River's devoted brother Simon (Sean Maher) manages to rescue her from the Alliance's clutches, but not before she learns a terrible secret about their oppressive rulers. For the past few months, the siblings have found refuge on Serenity, even though the ship's stoic captain Mal Reynolds (Nathan Fillion) doesn't exactly appreciate their presence. Now the Alliance has made one last-ditch attempt to find River, employing a nameless operative (Chiwetel Ejiofor) to hunt down and kill the girl and anyone who is sheltering her. Mal and his crew must decide if they are going to continue to run or stand their ground and make sure the truth is heard.

Whedon doesn't drop new viewers into this universe completely unprepared. The first 15 minutes are given over to bringing the audience up to speed on what has gone before and handy bits of exposition are strewn throughout the rest of the picture. It is entirely possible for someone who has never seen "Firefly" to follow and even enjoy Serenity. But it is ultimately unlikely that the film will resonate with moviegoers who aren't already invested in these characters. Viewed in the context of the show, the film's various revelations and tragedies pack a certain emotional punch. (At the screening I attended, "Firefly" fans could regularly be heard gasping or sniffling and sometimes both at the same time). On its own terms, however, Serenity doesn't clearly establish why these characters-or their mission-should matter to us. The storytelling is oddly slack as well; there's little sense of urgency to the crew's flight, largely because the enemy pursuing them isn't particularly well-defined. (The pacing does improve significantly once River's secret is finally revealed.)

In many ways, Serenity plays like a two-hour series finale rather than a standalone feature. While there's nothing inherently wrong with that approach, it does limit the film's potential to become the next big sci-fi franchise that Universal, Whedon and the show's fan base are clearly hoping for. Look for the movie to do its biggest business on DVD, when fans can purchase the inevitable two-disc special edition to place on the shelf alongside their "Firefly" box sets.

-Ethan Alter