For fans of the 2002 TV show "Firefly," the big screen spin-off, "Serenity," is easily a four-star episode packed with writer-director Joss Whedon's trademark themes: empowered women, consequences of actions, sacrifice, the need to believe in something bigger than yourself, and love.

	Sidney Baldwin Nathan Fillion plays Capt. Malcolm Reynolds in "Serenity." Click photo for larger image. Serenity Rating: PG-13 for sequences of intense violence and action, and some sexual references. Starring: Nathan Fillion. Director: Joss Whedon. Passion, luck energize creator of 'Serenity' Post-Gazette Family Film Guide review of 'Serenity'

F or science fiction fans who appreciate a semi-cerebral action-adventure and who have the background of other sci-fi universes to draw from to put together the unconnected pieces in the "Serenity" story, the movie should be solidly three-star satisfying.

For anyone else, "Serenity" is an overstuffed, exposition-heavy meanderer with limited appeal.

It's nearly impossible to take a TV series with nine regular characters and a 13-episode back story and squeeze it into a film that will satisfy die-hard fans and win new converts. At first, it seems as if Whedon might pull it off.

"Serenity" begins with a clever introduction that fills in the back story of River (Summer Glau), a -- literally -- tortured teen who's been freed from government captivity by her brother, Simon (Sean Maher). The pair hide from authorities on a spaceship called Serenity, a Firefly-class vessel captained by Mal (Nathan Fillion), a Han Solo-ish character.

"Serenity" never fully explains what the crew members do on a regular basis, but fans of the series know they're a transport-for-hire -- including illegal odd jobs -- who try to steer clear of the Alliance, this universe's Big Bad Empire.

In "Serenity," the Alliance baddies are embodied by The Operative (Chiwetel Ejiofor), who's out to nab River. There are also the mysterious Reavers, wild men who rape, pillage and defile with gluttony. Our heroes eventually take on both, which is probably one too many for a two-hour story to absorb.

As a stand-alone piece of entertainment, "Serenity" tries to serve fans and non-fans, but only fans (myself included after giving "Firefly" a second look on DVD) can fully appreciate its thrills, shocks, heartbreak and further developments in the aborted "Firefly" story.

"Serenity" drags a little in the middle, but it pulls together in the final quarter once it's clear how high the stakes are. The film benefits from Whedon's humorous dialogue ("We may experience some turbulence," Mal warns his crew during a bumpy landing, "and then explode"). And the sense of moral clarity he allows Mal to uncover gives the film's lead character a clear arc of growth.

But there's also fuzzy logic. In one poorly edited plot turn, it's unclear which set of bad guys are attacking the heroes.

Anyone who doesn't know the characters' back stories will be hard-pressed to care about what happens to them in the movie given the limited screen time most of them get.

"Serenity" marks a shaky big-screen directorial debut for Whedon, whose love of his characters may have blinded him to the impossible task of making a failed TV show into a movie.

It took two movies before "Star Trek" got its film formula right, so there could be hope for this franchise, but "Star Trek" benefited from being a comparatively simple show.

Whedon is a writer of complex characters and intricate stories better suited to a 22-episode TV season than a two-hour big screen movie -- or at least not a big screen movie based on a TV series. His real merit as a feature director won't be clear until he initiates an original film project that's not dependent on his celebrated TV past.