When it comes to Byzantine scheming and plots, those ancient Eastern Roman Imperialists had nothing on the Japanese yakuza.

The back-stabbing, front-stabbing, neck-snapping, hit-man/hit-woman intrigues of the new thriller “Hell Dogs” bears that out. It’s a movie of huge, murderous gangs and undercover cops killing their way to the top of them, of betrayals and corrupt Christian cults tied to “far right American politics,” of drugs and prostitution and ivory smuggling and homoeroticism.

And if it doesn’t simply wear you out, it’s not for lack of trying. The opening act’s mountain of exposition, skipped-through backstory and tsunami of names of mobsters and molls and allegiances and complications are a LOT of clutter to plunge through, especially on a movie Netflix hasn’t gotten around to creating an English language soundtrack for.

Lots and lots of subtitles, so don’t say you weren’t warned.

It’s damned near impossible to figure out who is relating to whoever and wherever this unwieldy beast is set.

But focus on the performance of Jun’ichi Okada, who has aged out of his J-pop years into a properly grizzled screen samurai, cop or brawler who punches above his (modest) height. He is a riveting presence who holds this movie together when it is talking and back-storying us to death, just the guy you want to answer the question “Why?” near the end.

“You want the long or the short version?”

We meet him as an unassuming, muscular slob strolling into a rural chicken farm, identifying a former yakuza (Japanese mafia) killer named “Mad Dog.” He’s there to expose, confront and with a shrug, suggest “You must atone” for that tattooed body-count of kills inked onto his arm.

But the guy making this suggestion has a body-count tattoo, too. Mad Dog never knew what hit him.

It turns out this avenger’s name is Goro Idezutsi (Okada), and he used to be a cop nicknamed “G.I.” He’s long off the force, just a man wandering the land like a samurai, settling scores. An undercover unit’s chief (Yoshi Sakô) grabs him, shames him with “No cop has fallen lower than you,” and offers him a deal — a well-paid undercover job to infiltrate one of the country’s most mob-infested regions.

They’ve got computer personality read-outs on which young made mob man he’d be best suited to partner up with, Muro (Kentarô Sakaguchi). He’s to work his way into the elite squad of the Kozu family, the Hell Dogs. He will have to kill his way to the top by wiping out “The Expelled” (outcasts forming their own gang) and bringing down those top gangsters who have gotten cozy with the police.

As “Tak,” his undercover alias, he will mix and flatter and impress Pops, Bear, Nas-Teeth, Slick and others, working his way towards towards the runway-model slim pretty boy in charge, Toako, played by pop star/actor Miyavi.

And if Pops’ woman, Emiri (Mayu Matsuoka), takes a shine to him, he’ll just have to finesse his way around that, too.

It takes a good, long while for this narrative to weed through the early slaughter — a prologue that sets-up what is being avenged — and get to a point where it’s lean enough to grasp. But the charismatic Okada drags us along as we see double and triple crosses, mass murder and fights involving all sorts of ordnance — a 1935 Manville revolver shotgun, for instance, just the thing you want when you’ve become this or that boss’s bodyguard and they’ve wandered into an abandoned factory trap, with platoons of rival gangsters storming in, wearing face shields and rain slickers to keep from getting all splattered with blood as they shotgun everyone in sight.

“Hell Dogs” is a deep dive into yakuza genre thrillers, featuring locations Westerners never see (ruined bottling plants, etc), exotic, pimped wing-doored SUVs that Japan doesn’t export and police ethics (killing your way through a mob) not common in Western cinema and Western policing.

Writer-director Masato Harada (“Baragaki: Unbroken Samurai”) and his fight choreographers manage to up the ante, brawl after brawl, shootout after shoot-out. These escalate, sometimes in scale — sheer numbers — sometimes in simple shock-value and intensity. One involving a hit-woman made my jaw drop.

It’s not a film for the faint-hearted, with torture and yakuza finger-lopping and fights to the death so intimate we feel the high stakes and desperation.

But it’s also goofy. The gangsters speculate on Toake’s sexuality. I mean, who wouldn’t? But there’s no judgement here, because when you’re naked around other guys so much you’ve memorized their full body tattoos, who knows who swings which way? You’d be surprised. You will be surprised.

Mobsters, as in accurate depictions of their “class” in other countries, are seen as gauche, callous and stupidly careless. One higher-up makes his karaoke choices straight out of the Andrea Bocelli songbook. Another drags his wife onstage to duet the workers of the worlds anthem, “L’Internationale.” Damned commie. Probably Chinese. These yakuza have their fingers in lots of countries and all sorts of illicit trade.

“Hell Dogs,” whose full title is “Hell Dogs: In the House of Bamboo,” is a lot to take in and a movie that takes too long to give us our bearings.

But our tour guide through it all is the dogged, scowling, undeterred Tak, aka “G.I.,” a hard man with a secret inside of a ploy buried behind a bigger secret. Okada’s violent world-weariness in the part makes this guy the only one who, in mid-slaughter, can give anybody on screen or off, the answer to “why” all this is going on.

“You want the long or the short version?” Let’s take the long one and see how often it makes our jaws drop.

Rating: TV-MA, graphic violence, nudity, profanity

Cast:Junichi Okada, Kentaro Sakaguchi, Mayu Matsuoka, Miyavi and Yoshi Sakô

Credits: Scripted and directed by Masato Harada, based on a novel by Akio Fukamachi. A Netflix release.

Running time: 2:18