Starring: Nathan Fillion, Gina Torres, Alan Tudyk, Morena Baccarin, Adam Baldwin. Rated PG-13.

Joss Whedon doesn’t give up easily. When his 1992 teen comedy Buffy the Vampire Slayer fell on its face at the theaters, he reinvented it five years later for television, and, in doing so, gave birth to one of the most enduringly brilliant franchises in the history of the format. Now, he’s making his feature-length directorial debut with Serenity, a sort-of sequel to his short-lived 2002 series Firefly, unceremoniously dropped by FOX after one promising season. That’s good news for the show’s cult-like followers, who made the Firefly DVD set enough of a commercial hit that Universal green-lighted a cinematic treatment. It’s also good news for everyone else, because Serenity is a first-rate adventure, driven by Whedon’s fertile imagination and slicing wit.

Is it visually groundbreaking? Hardly. Unlike fellow sci-fi nerd George Lucas, Whedon is less interested in expensive special effects than in rich, character-driven drama, so if Serenity’s cheesy sets seem a bit retro, he can be forgiven. None of his celebrated TV shows have ever looked very good, but that wasn’t the point. His strengths have always been his ear for sharp, humorous dialogue and his ability to inject genuine humanity into stories about robots, vampires, demons and futuristic space cowboys – qualities that the Star Wars guru sorely lacks.

Fans of Firefly will be familiar with the background details, but Serenity stands on its own well enough for the uninitiated, even if its character development is somewhat abbreviated. The story: Mal (Nathan Fillion) is the captain of the starship Serenity and its crew of merry thieves, from the thuggish Jayne (Adam Baldwin) to Zoe (Gina Torres), his more thoughtful second-in-command. When they assist their resident doctor, Simon (Sean Maher), in the daring rescue of his sister, River (Summer Glau), they become the targets of an intergalactic manhunt.

River, it turns out, isn’t your average 17-year-old. She’s a psychic with a mean right hook who’s been turned into a human time-bomb by the Alliance, the dark overlords of space. When Mal makes off with their favorite toy, who just happens to carry a secret that might topple their bureaucratic empire, they dispatch a vicious assassin (Chiwetel Ejiofor) to track her. An epic, two-hour chase ensues, culminating in a rousing battle that pits Mal and company against the Alliance’s steely operative and an army of flesh-eating killers known as Reavers.

It’s a simple enough story, a variation on a time-tested Star Trek riff, but it works so well because of its clever script, breakneck pace and self-assured performances, particularly by Fillion. He is a natural leading man who presides over his crew with an endearing mix of compassion and sly humor, underlined by the unquestioned toughness and loyalty that all good leaders must possess; he’s also the heart and soul of Serenity.

So sure, it’s a western set in space, populated by characters who occasionally curse in Mandarin. (Again, it works. Seriously.) For all of its inherent silliness, Serenity is riveting stuff, one of the smartest and most humane sci-fi adventures of our generation. And, as directorial debuts go, Whedon’s is a smashing success. -- Rossiter Drake