As Eugene Jarecki’s recent documentary “The House I Live In” demonstrated, US drug laws—and their enforcement—are in many respects a scandal, and “Snitch” deserves credit for dramatizing that unhappy fact. It’s a pity that writer-director Ric Roman Waugh and co-writer Justin Haythe have chosen to send their message in a movie that’s a weird cross between Liam Neeson’s “Taken” pictures and one of those Lifetime cable movies in which a mother will do anything to save her child from being wrongfully convicted for some heinous crime.

Dwayne Johnson stars as John Matthews, the owner of a construction firm whose estranged teen son Jason (bland Rafi Gavron) gets arrested for dealing in Ecstasy. When he learns that the boy is facing a long prison term under mandatory federal sentencing guidelines, John decides to infiltrate the local drug culture to get the goods on a kingpin and give the information to Joanne Keeghan (Susan Sarandon, doing her icily businesslike routine), the hard-nosed prosecutor who promises to reduce the boy’s jail time in in return.

The undercover scenario is old hat, of course—probably the best modern variant was the CBS series “Wiseguy,” which handled it really well. So’s the motivation—saving your child from jail. Still, “Snitch” would work better if it didn’t stack the deck in such crude, obvious ways. Jason isn’t really a drug pusher at all—he’s just a naïve kid set up by his best friend, who’d been caught by the feds and promised a sweet deal if he delivered somebody—anybody—else for them to arrest. And when Matthews delivers a real dealer named Malik (Michael K. Williams) to her on a silver platter, Keeghan—who’s also running for Congress, of course—reneges, demanding that he move up the chain to cartel leader Juan Carlos “El Topo” Pintera (Benjamin Bratt) instead. It’s a good thing that John’s company has a fleet of semis, which can be used to transport mountains of cash across the border to Mexico. Cue the big freeway chase in which his truck is pursued by a bevy of bad guys in their cars, guns blazing, while Keeghan’s right-hand man, Cooper (Barry Pepper, sporting an unsightly goatee and looking more like Robert Patrick than ever) speeds to pick up El Topo. (The actual arrest, in which Cooper coincidentally spies his quarry in an SUV headed in the other direction, is so absurd that it elicits audience snickers.)

One can imagine a story of this kind being handled in a more subtle, and therefore more interesting way. Jason could have actually been involved in the drug trade, which would have made for some real moral complexity, largely absent when he’s just a dumb innocent. (The effort to make him seem “noble” for refusing to name somebody else to get a reduced sentence himself is strained.) And John could really have been a regular guy, something that hardly comes across by casting ex-wrestler Dwayne Johnson in the role. He actually gives his best performance to date, but certainly isn’t anyone’s idea of a meek little milquetoast called to action duty. Instead he’s way more muscle-bound than the villains he’s up against—he literally looks as though he could snap them like twigs—and it’s no surprise when he turns into a fairly typical action hero. Keeghan could have been portrayed as something other than what another character calls a “dragon lady,” too.

Still, “Snitch” has some strengths. Working with production designer Vincent Reynaud, art director Joe Lemmon and cinematographer Dana Gonzalez, Waugh manages to capture the gritty underbelly of the unnamed Missouri city in which it’s set quite satisfactorily. He’s helped by the effective performances of Williams as the reclusive Malik and Jon Bernthal as the ex-con Matthews persuades to act as his introduction to the drug trade (the character might be stock, but Bernthal invests it with authenticity). Bratt cuts a cool, unflappable figure as the cartel honcho, and Pepper an appropriately tough one. But Merlina Kanakaredes, as Jason’s mother, and Nadine Velasquez, as John’s second wife, are pretty much wasted. This is a macho enterprise—even Sarandon swaggers.

“Snitch” earns points for dealing with a serious social issue, and for offering Johnson the chance to display his improving skill as an actor. But by trying to pass muster as a conventional action movie while delivering its message, it comes up a mite short.