Riddick has become the unwilling leader of the Necromonger cult. Betrayed into thinking that his home planet, Furya, has been rediscovered, he's abandoned on a deathworld, and attracts the attention of two groups of mercenary bounty-hunters - one simply out for the reward money, the other led by a man with a personal agenda - in an attempt to effect his escape. It's a simple but effective three-act story in which Riddick and the franchise rediscover their mojo. In the first part, Riddick makes like a barbarian version of Robinson Crusoe on Mars, battling alien predators and setting up camp; in the second, the viewpoint shifts the mercenaries as Riddick picks them off one by one; in the third, as in Pitch Black, the surviving players have to deal with a horde of monsters.
The tension occasionally slackens as the mercenaries squabble and wander about the landscape; the lone significant female character (Katee Sackhoff), despite her demonstrable toughness, is defined by male attention (and given an unnecessary shower scene); oblique references to previous films may confuse those who haven't seen them. But there's plenty of good hard-edged dialogue, and Vin Diesel completely inhabits his anti-hero, setting his intricate plan for escape in motion, ghosting in and out of darkness and rain, and calmly pulling off a startling execution while chained up.
It's all good, old-fashioned B-movie fun, punched up with gory effects. And while it doesn't give any new insights or character development to its anti-hero, it's a shot in the arm for the franchise and a refreshing change from this summer's depressing parade of bloated blockbusters.