With Arnie busy playing governor in Sacramento, someone's got to fill the void left for muscle-bound automaton heroes. Cue Vin Diesel in this sci-fi action flick, a sequel to 2000's Pitch Black The body count almost reaches Schwarzeneggerian levels yet the concept falls short. Here's hoping Diesel will someday latch on to a Terminator-like story that stimulates the mind and the senses. For now it's all galactic chases and deadly knife fights when his 26th-century outlaw must prevent a fascistic sect leader from wiping out the human race.

The movie opens with Judy Dench intoning ominous narration about the only guy able to save mankind. The next voice you hear is Diesel’s and the contrast between these two sounds couldn’t be starker--which is not to say jarring or incongruous. Both actors are coolly self-assured in their way, and it’s actually too bad Dench and Diesel don’t get substantial screen time together.

Dame Judy is not completely off the hook however. She's still slumming it as an ethereal soothsayer who aids the goggled hero. Riddick belongs to a breed of warriors called Furians. Judging by Diesel's physique, they believe in the liberal use of steroids. He's got a bounty on his head and funky white eyes that are ultra sensitive to light and allow him to see in the dark. We don't learn how this came to pass and unless you're a devotee of Pitch Black it's not easy to grasp many of the background details. Nor is it necessary for enjoying this exercise in mortal combat.

Every life form, especially the human race in all its diversity and pluralistic splendor, is threatened by Necromongers led by the viscious Lord Marshal (Colm Feore). He's in the business of literally snatching souls and converting those he conquers to his religion through a painful and not very effective brainwashing process.

Probably because it began as a video game, this scenario lacks substance or grandeur. Clumsy labels like Necromonger, Elementals, Underverse, and World-Enders are bandied about.  If there's any subtext it's in the theme of imposing one's faith on others. Writer-director David Twohy doesn't muddle things with intricate plotting or dialogue. Logical, since it's for video gamers whose synapses are trained for quick reflexes. Yet with that in mind he could have trimmed a few minutes off the running time.

The special effects are adequate. The best sequence occurs on a penal planet where the temperature reaches 700 degrees and outrunning the rising sun is a matter of life or death. Regarding the art direction, why is every futuristic civilization envisioned as a jumble of high- and low-tech? Here it's a way of illustrating the Necromonger's so-called "10th Crusade," so most everyone is dressed like mediaeval jousters and wields crude weaponry like scythes and knives. There’s also S&M and crucifixion imagery to bolster the motif.

Diesel delivers one-liners that are part and parcel of being an action hero. "It's been a long time since I've smelled beautiful," he says when getting a whiff of a Lady Macbeth figure played by Thandie Newton. And when soothing a savage creature preying on convicts, he explains, "It's an animal thing." Yep.

(Released by Universal Pictures and rated "PG-13" for intense violent action and some language.)