Dirty cops, corrupt prosecutors and tainted politicians collide in “Yadang: The Snitch,” a twisty thriller about how drugs not only kill people, they stain every corner of The State that they touch.

The debut feature of Hwang Byeong-guk is built as a star vehicle for “Squid Games” phenom Kang Ha-neul — all smiles, style and swagger as the title character, the “liaison” between law enforcement and the drug users, dealers and kingpins they pursue.

A cluttered opening keeps the picture from finding its footing quickly, and a rather infantile “Let’s explain what just happened to slower viewers” antic-climax robs the finale of some of its sting.

But it’s well-cast, and well-acted and the plot, once we get hold of it, has plenty to keep the viewer engaged.

Lee Kang-soo (Kang) is a well-compensated snitch who agreed to this dangerous work after being set-up by a customer in his previous profession, ride-share driving.

Ambitious Prosecutor Goo (veteran character actor Kim Geum-soon) knows Kang-soo was set up. But there’s no retrial or freedom for him without a “deal.” He starts informing on inmates running a drug gang from inside prison, wins his freedom and soon tools the means streets in a luxury SUV, naming names, fingering middle men and women and helping the prosecutor rise through the ranks.

Prosecutor Goo (“Ku” in the subtitles) is even moved to call Kang-soo “bro.”

Our snitch doesn’t have much of an interior life. But if he has a rationalization for what he does, it might be what he relates to one hapless victim.

“The world’s longest-lived junkies are Korean (in Korean with English subtitles). Because the cops lock you up before you OD!”

Another cog in the machinery of law enforcement is two-fisted cop, Det. Oh (Park Hae-joon of “12.12: The Day”). He busts a young actress, Uhm Soo-jin (Chae Won-bin) with drugs on her and turns her into another snitch.

But her connection to hard-partying, high-living Cho Hoon (Ryu Kyung-soo), the son of a presidential candidate, could be everyone’s undoing. The favors the prosecutor doles out, the pecking order of drug bosses, the detective’s ongoing investigations and the presidential race could turn on who double-crosses whom, and who figures out they need to team up in order to expose the wrongdoers, and survive exposing them.

Screenwriter Kim Hyo-seok, who adapted John Woo’s “A Better Tomorrow” for a Korean remake, fills the middle acts of “Yadang” with intrigues on top of intrigues — a “French Connection II” twist (turning your victim into an addict), new drugs from North Korea, gangsters making deals to get rivals taken down and secret videos that could be a lot of people’s undoing.

Kang vamps up his character, waggling his cigarette with his teeth, taking beatings, grinning and laughing off the perilous spot his snitch is in.

Park’s detective serves up Six Degrees of Outrage as he finds himself outflanked by crooks and a prosecutor with his own “special unit” set up to serve “justice” and his unhinged ambition, and not in that order.

Chae Won-bin’s role is painfully underwritten, the curse of many a macho Korean thriller.

But the various subplots collide in entertaining ways, and the “payback” chapters are full of surprises, which are easy enough to understand without the tedious business of throwing in anti-climactic flashbacks to ensure everybody “gets” why this or that happened and why any of it makes sense.

We got it. We were paying attention.

Rating: unrated, graphic violence, drug abuse, sex, profanity

Cast: Kang Ha-neul, Yoo Hae-jin, Chae Won-bin, Park Hae-joon, Kim Geum-soon, Yoo Seong-ju and Ryu Kyung-soo

Credits: Directed by Hwang Byeong-guk, scripted by Kim Hyo-seok. A Well Go USA release.

Running time: 2:02