Who knew that police sting operations did — or even could — work like the one seen in “The Stranger”? Based on the extensive Mr. Big ruse that brought a notorious Australian kidnapper to justice, this eerie, understated thriller draws the audience into the same deception used to ensnare the culprit, focusing on psychology more than procedure in its entrancing account of a most unusual criminal investigation. The case echoes Denis Villeneuve’s “Prisoners” at times, minus the twisted genre-movie payoff, which will limit commercial prospects beyond Oz shores.

With a tortured performance by producer-star Joel Edgerton at its core, this second feature from gifted actor-turned-helmer Thomas M. Wright is as much about befriending the devil as his terrific 2018 debut, “Acute Misfortune,” was. (That film re-created the moth-to-flame portrait of a self-destructive bad-boy artist by an easily manipulated young journalist. Here, an undercover cop must cozy up to an unrepentant human monster.) Premiering in Un Certain Regard at Cannes, “The Stranger” confirms that Wright has arrived, even if his treatment sometimes feels more oblique and self-consciously arty than the material demands.

Pretty much everyone in Australia knows the case of Daniel Morcombe, the 13-year-old boy who disappeared from a Queensland bus stop. Like last year’s Cannes competition entry “Nitram,” about the country’s worst mass shooting, downplaying the crime itself is a way of depriving the real-world perpetrator of the toxic brand of tabloid celebrity that American media and movies so often confer upon evildoers. But the tactic doesn’t erase the evil itself, while non-Australian audiences may be left somewhat confused by the omission of key information.

Here, for instance, Wright never reveals how the police identified “Henry” (Sean Harris) as the prime suspect in a nationwide manhunt. As far as we’re concerned, he’s just a weary, working-class drifter we meet on the bus at the same time a guy named Paul (Steve Mouzakis) does. These two travelers hit it off, then go out for drinks. Paul tells Henry he has a lead on some work, and sensing that it might not be entirely legal, Henry quickly volunteers, “I don’t do violence.”

Neither does Wright, it turns out. The director resists the temptation to show whatever darkness Henry is hiding, working instead to suggest the unknowability of another man’s soul. Ergo, “The Stranger” becomes an exercise in mood, as characters tend to be seen in shadow, from a distance or with their backs to the camera, while poker-faced expressions mask their emotions.

A few scenes in, the far-from-exploitative film’s point of view shifts dramatically, from Henry to the rough, bearded guy — another stranger (Edgerton), this one named Mark — who picks him up for the job the next day. At this point, viewers are still getting their bearings, letting their imaginations run wild based on limited information. “Trust Mark,” Paul tells Henry. “If you’re honest with these guys, you’ll be looked after.” Trust is a key theme in “The Stranger.”

Edgerton has played criminals before, of course, perhaps most memorably in 2010’s “Animal Kingdom,” and here, we’re invited to wonder what kind of man his character is. Not at all the one the movie first suggests, it turns out. Mark is decent, but not without demons of his own. The film follows him home … to his son, whom Mark has been raising by himself. He’s an undercover cop, it turns out, who spends his days embedded in some kind of mob-like organization. It’s not easy to shake that persona when he’s off the clock — as if he’s ever truly off the clock — and so he unwittingly takes it out on his kid.

Meanwhile, his job is to win Henry’s confidence, to extract a confession. Everything else is a lie, an elaborate charade by which to entrap Henry — who isn’t Henry at all, but a man named Peter Worley. But how do you get a man who won’t assume his own guilt to unburden his conscience to a man he barely knows? Already, this review has said more than it should about “The Stranger’s” secrets, which are best discovered as they unfold. The impact will be inevitably diluted for those who don’t know the backstory of the Morcombe case, since Sean Harris, the wild-eyed English actor who plays Henry/Peter, comes across as a sympathetic character at first (audiences might think otherwise if they knew what he’d done).

In the U.S., U.K. and several other countries, the police can’t use such schemes to elicit a confession, which makes the whole film — adapted from Kate Kyriacou’s rigorously reported book, “The Sting” — a compelling illustration of how the resource-intensive strategy worked in Australia. (It’s no small thing to invent a fictional criminal organization just to ensnare one man.) There’s a simultaneously fascinating and frightening stretch in which the cops have the testimony they need but not the evidence to prove it in court.

The final scene of “The Stranger” follows Mark home, and the point of view shifts again, this time to his son as he studies the effect the investigation has had on his dad. That’s a strange way for Wright to wrap a movie that, like “Zodiac,” leaves its detectives transformed by the experience, effectively making the case that seeing the human side of evil changes a person — which of course, is what Wright intends for his movie to do to us as well.