The shortest distance between two points is popularly believed to be a straight line. But if one of those points is the chin, cheekbone or torso of some sneering and/or psychotic Korean gangster, the shortest route is actually the arc described by either one of Korean megastar Don Lee‘s fists, here playing the fists of Detective Ma, protagonist of the ludicrously watchable “Roundup” series. 

“The Roundup” and “The Roundup: No Way Out” the first two sequels to 2017’s “The Outlaws,” seemed to inch a little closer each time to the platonic ideal of star-driven action-franchise purity. Now, with Lee (aka Ma Dong-seok) and his fists directed by Heo Myeong-haeng, a stunt choreographer who worked on Lee’s breakout “Train to Busan,” the fourth episode gets closer still. “The Roundup: Punishment” minimizes unnecessary originality, while gloriously maximizing the opportunities for Lee to crack wise, or look aggrieved and a little bored, as though he’s just remembered he needs to do laundry, all while his meaty forearms land a flurry of sledgehammer punches so rapid their recipients, often quite literally, do not know what hit them. This, truly, is cinema. 

To date, Detective Ma’s appealingly tight-knit Metro Investigations squad have taken on loansharks, kidnappers and drug peddlers. This time out, the knife-murder of a young Korean national puts them on the trail of an online gambling racket run through the Philippines by ruthless, blade-happy sicko Baek (Kim Moo-yul). Baek’s boss, a weak-chinned piece of work called Chang (Lee Dong-hwi), not content with raking in and laundering zillions though his casino empire, is looking to launch his own cryptocurrency. All of which cyber-skullduggery provides plenty of jokes at the expense of befuddled digital dinosaur Detective Ma. 

Ma might hear mishear the “cloud” of “cloud storage” as “crowd” and imagine that a mob of people will somehow be involved in syncing his contacts to a new phone, but he’s no dummy: It takes a smart guy to know what he doesn’t know. So after adding a couple of cybercrime experts to his task force, he settles down to errands more in his wheelhouse, such as pulling gates off their hinges, laying waste to airplane first-class cabins, and making knuckly contact with a lot of soft tissue. Well, it’s soft after its encounter with Ma, even if it wasn’t before — most of his opponents, especially Baek, are sleek and ripped and a pleasure to see pummelled into pulp. 

Along the way, Ma reunites with series-regular Jang I-su (Park Ji-hwan) a petty hoodlum with an attachment to his designer man-purse and a secret desire, which Ma exploits amusingly, to be accepted as one of the squad. But the plot, which like all the “Roundup” stories including the (hallelujah!) four more in development, is very loosely based on a real case, is just a pretext. It’s the flippers on a pinball machine that can send Ma pinging fist-first from one baddie to another, without ever making us wait too long in between encounters. When one non-fighty interval threatens to go on a bit, Oh Sang-ho’s script contrives a completely unrelated fracas to kick off just as Ma happens to be passing by in a disgruntled mood.

Heo Myeong-haeng proves to be a very efficient action director, and DP Lee Sung-je has an unerring instinct for placing the camera where it can best exploit the contrast between the speed of Lee’s movements and the resigned stoicism of his expression. Not to mention that the clever, mobile framing implies a whole bunch of gore without ever directly showing any sliced jugulars or gaping chest wounds. But if there’s a below-the-line standout craftsperson it’s probably sound designer Gong Tae-won, who manages to foley consistently satisfying punch noises, presumably by flinging sides of beef off tall buildings and recording them as they hit the ground. Without these meaty sonic mnemonics of full-flesh contact, it’s possible our brains would misread the hyperfast blur of Lee’s upper body as total inaction, and fancy the split lips and livid contusions blossoming on his victim’s face were somehow happening spontaneously.

Lee was one of the best things about Marvel’s “Eternals,” but his bid for Hollywood stardom still neutered the charisma that entirely powers this purpose-built, made-to-order franchise. Take him dispensing a zinger like “What are you going to do, spread jam on me?” when his adversary grabs a nearby butter knife. Far from the snickering John McClane model, Lee’s delivery sounds less like a taunt than a gently dismayed rebuke. He’s not angry with his opponent. He’s disappointed. As well he should be, given that Korea’s laws put firearms all but out of the equation and knives are the next-most-cowardly cheat to introduce into a fistfight. Don’t these baddies-of-the-week know? When it comes to weaponry, Don Lee’s gun show is all anyone will ever need.