Spiral

Feats of mechanical engineering outshine those of narrative originality in Spiral, a lackluster spinoff from the Saw torture-porn franchise.

Amid the usual sadistic collection of gadgets and game play that prey upon the squeamish, the unusually cast Chris Rock brings depth and complexity to a conflicted character who deserves better than this procedural thriller.

Rock plays Zeke, a brash detective at an urban police precinct once run by his father (Samuel L. Jackson), an ex-chief whose no-nonsense tactics sometimes raised ethical questions and left his relationship with Zeke fractured.

The department stunned when one of Zeke’s colleagues is found brutally murdered. As the new chief (Marisol Nichols) appoints Zeke and his rookie partner (Max Minghella) to lead the investigation, initial clues point to a copycat of Jigsaw, the serial killer who was finally killed off in the prior franchise installment.

It’s not long before other cops are targeted, and the killings become more brazen, apparently as retribution for alleged corruption. And for Zeke, whose reputation already has been damaged, the case becomes personal.

As directed by Darren Lynn Bousman, who returns after helming three of the — checks notes — seven Saw sequels, this installment delivers intermittent doses of the gore that series fans expect. A grisly opening sequence in a train tunnel, for example, maximizes the discomfort for those both on-screen and off.

Never more than mildly suspenseful or scary, the screenplay is more ambitious and plot-driven than many of its predecessors, but the abundance of logical gaps diminishes the credibility. After all, these movies haven’t been about caring who dies, but rather how they meet their demise.

In addition to showcasing his range, Rock also digs into his wheelhouse for some comic relief, including an early rant about how Forrest Gump would be reinterpreted in today’s politically correct climate.

It’s too bad that broader awareness doesn’t carry over to the film as a whole, which awkwardly fits into the volatile contemporary landscape in terms of law enforcement and social justice.

At any rate, the allure has worn off the franchise to the extent that even a reinvention such as this feels like little more than a desperate cash grab — or the continuation of a downward spiral.

Rated R, 93 minutes.