If after eight movies in the "Saw" franchise — the new "Spiral" makes nine — you're still unfamiliar with or (harder to believe) undecided on the concept, let me get this out of the way up front: The movies center on the gruesome infliction of pain, perpetrated as a means of teaching a lesson, though more often the "student" just ends up dead, often minus a body part or two.

Typically referred to as "games," the scenarios are elaborate and theatrically violent, though usually shorter in duration than one would find in torture porn, a genre into which some have lumped the films, though that isn't exactly right.

"Spiral," which involves the hunt for a serial killer by the police force of a nameless metropolis, is a thriller, a mystery, a police drama, but it hews closely to "Saw's" grisly curriculum. Detective Zeke Banks (Chris Rock) has stumbled onto a series of murders that appear to be copycat killings perpetrated by someone who has adopted the methods of the "Saw" movies' antagonist: the now deceased John Kramer, aka Jigsaw (played by Tobin Bell).

The film opens with the death of Banks's pal, a fellow detective (Dan Petronijevic) who has been lured onto a subway track, where he is strung up, essentially, by his tongue, in the path of a speeding train. If he's willing to sacrifice that organ — a metaphor for the lies he's told in court, according to the message from a man in a pig mask playing on a nearby TV — he lives.

Things don't quite work out that way, in the first of a series of grisly demises, all involving police officers or former officers, and entailing, among other things, flaying, a hailstorm of broken glass shards, hot wax and finger avulsion. (Look it up. On second thought, don't.)

So the film, directed by Darren Lynn Bousman ("Saw II" through "Saw IV") from a screenplay by Josh Stolberg and Pete Goldfinger, and executive produced by Rock, has a topical jolt: Someone is punishing rotten police officers. Police officers who lie, abuse their authority, shoot innocent people and cover up for them. That makes it ... good? It certainly gives it a bit of much-needed urgency anyway, this deep into a franchise that was starting to show signs of age.

Zeke is, presumably, one of the good ones. He turned in a corrupt fellow officer once, which has made him a pariah. As the film opens — with an undercover job incorporating a riff on the movie "Forrest Gump" that plays like a Rock stand-up routine — he's been assigned a rookie (Max Minghella) to mentor, as a way of making him more of a team player.

Further backstory: Zeke has a complicated relationship with his father (Samuel L. Jackson), the former police chief. Everyone else seems to openly hate Zeke. Half of them are suspects, and the rest potential victims.

Stylistically, the film looks good, shot in a palette of grimy browns that give it the patina of sweat-stained leather. The rare pop of color is delivered via a series of Tiffany-blue packages containing clues, and the bright red spray-painted spirals left by the killer — a symbol of growth, change and evolution, we're told, that is ironically largely missing from the film. Yes, the killer leaves taunting clues, making the film feel like something of a throwback — a sense that is underscored by all the electric fans humming in the background of every other scene, like a movie made before air conditioning.

But the story itself is kind of atavistic, too. You may find that you have pegged the killer early on, which means there's something wrong with the plot, structurally — the twist isn't much of a twist — or you've seen too many of these things. Possibly both are true.

This inevitable decline raises the question: If you've managed to avoid 17 years worth of "Saw" movies so far, why start now?

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"Spiral." Two stars. Rated R. At theaters. Contains sequences of grisly, bloody violence and torture, pervasive crude language, some sexual references and brief drug use. 93 minutes.