Imagine if Warner Brothers had locked up their vaults 20 years ago. Nothing on video, no films in retrospectives, no prints provided for scholars — for all intents and purposes they just ceased to exist. There’d be rumors about legendary actors with names like Bette Davis and Humphrey Bogart, but as time passed memories would fade and they’d wind up as shadowy figures on bootleg tapes of weird old movies like “The Maltese Falcon.”

This is exactly what happened to the Shaw Brothers, the world’s last great major motion picture studio, and it’s exactly why BAMcinématek’s “The Shaw Brothers Return: Heroic Grace II,” which continues a BAM series from 2005, is nothing short of a miracle. Between 1957 and 1985, the Shaw Brothers were Hong Kong’s largest film studio, with their glitzy musicals, comedies, and romances minting money in theaters from Bangkok to Taiwan. But what they’re best remembered for are the martial arts flicks that made Cheng Peipei, Ti Lung, and Jimmy Wang Yu household names across Asia. A series of boneheaded business decisions — they passed on putting both Bruce Lee and Jackie Chan under contract — and the rising costs of production forced them to shutter their doors in 1985, locking away their movie library and erasing three decades of Chinese film history.

In 2000,Celestial Pictures bought the Shaw Brothers library and began to rerelease these classic films on DVD and in retrospectives like this one, and what moviegoers encounter is an embarrassment of riches: too many stars, too many directors, and too many classic films all at once. Fortunately, BAM’s dizzying blast of Chinese cine-mania is anchored by three great directors: the bloody fist of Chang Cheh, the elegant sword of Chor Yuen, and the impeccable stances of Liu Chia-liang.

Chang Cheh was first. Emerging from the Hong Kong riots of the 1960s and overreacting to the Chinese tradition of movies starring weak men and strong women, Chang forged hypermasculine films in which men stripped to the waist and turned the screen into a slaughterhouse.”The Boxer From Shantung” (December 6) is simultaneously a Marxist tract written in blood and a Shanghai “Scarface”set in the 1930s that presents the upper classes with a bill detailing exactly what they have to fear from those who have nothing to lose. Like all Chang’s movies, “The Boxer” slowly tightens its plot until springing shut like a nasty trap in the final reel, unleashing an apocalyptic bout of bloodshed.

“The Five Venoms”(November 27) offers a crash course in corruption as a dying master sends his last, and worst, student out into the world to disband his clan. The problem is his students wore masks and can only be identified through their funky skills, like iron skin and wall walking.

Chor Yuen is the Douglas Sirk to Chang Cheh’s Sam Peckinpah, constructing elegant laments (shot entirely on soundstages) for innocence lost. In “Clans of Intrigue” (December 5), his gay detective lounges on a pimped out pleasure barge, sipping wine and reciting poetry until he’s accused of a triple murder. With 30 days to clear his name, he finds the whole thing a bit of a lark, giggling to himself as he romps around assembling clues that eventually lead to a sadistic stab-down in a gaudily lit underground pleasure grotto.

“The Jade Tiger” (November 14) is a classic revenge film that leaves the taste of ashes in your mouth. The oldest son of Dafeng Hall sets out to avenge his dad’s decapitation at the hands of the Tang Clan, but as the plot twists multiply, he’s forced to sacrifice his family, his friends, and even his wife in his single-minded pursuit of filial duty.

Martial arts movies are terminally allergic to happy endings, but Liu Chialiang did his best. The only real martial artist in this lineup, he mixed knockabout comedy with kung fu, which make his films seem like sparkling champagne after the claustrophobic, hermetically sealed worlds of Chang and Chor. The unfortunately named “Dirty Ho” (November 28) is a kung fu version of Shakespeare’s “Henry IV, Part I” and it’s the highlight of this retrospective.

A young prince loafs incognito in Southern China, appraising antiques and old wines before learning that he must fight his way back into the Imperial Palace in time for the naming of the new Emperor. A true epic, “Dirty Ho” reaches dizzying comic and martial heights before ending on a depressing freeze frame that reminds everyone that kings are kings and men are men and never the two shall meet.

“Dirty Ho” star Gordon Liu is known in the West for his role in Quentin Tarantino’s “Kill Bill,” but forget Tarantino’s shallow pop irony, and forget the cheap martial arts junk you saw on Saturday afternoon TV — those movies were low budget travesties, shot in Taiwan with actors so far past their primes that vultures circled the sets. They all have their roots in the rich, wild spectacle of the original Shaw Brothers films, and what BAM is offering is a heady sip of martial opera, taken straight from the source.

Through November 28 (30 Lafayette Ave., between Ashland Place and St. Felix Street, 718-636-4100).

The Shaw Brothers actors were sculpted of solid charisma and their performances will brand themselves forever on your brain. Here’re five to watch for:

Lo Meng in “The Five Venoms”

The muscleman of the five-man team of actors who became known as “the Five Venoms,” Lo Meng’s bluff heartiness as the Iron Skinned Toad makes his gruesome death truly horrifying.

Kara Hui in “My Young Auntie”

Imperious Kara Hui dishes out the pain like Marlene Dietrich with a mean roundhouse kick. And don’t miss her playing a male role in “Legendary Weapons of China.”

Alexander Fu Sheng in “Legendary Weapons of China”

The Shaw Brothers’ James Dean, he was supposed to be Jackie Chan before a car accident ended his life at 28. His cameo in “Legendary Weapons” as a con man hired to impersonate a retired martial legend sets the screen on fire.

David Chiang in “The Boxer From Shantung”

A vehicle for Chen Kuan-tai, this flick is stolen by Chiang as dapper gang boss Tan Si. Facing a fatal ambush, he adjusts his suit then breaks out in a delighted grin before wading into his final battle.

Ha Ping in “The Magic Blade”

Of Ping’s hundreds of roles, none is as much fun as the cannibalistic “Devil Granny” in this bizarro martial arts movie full of human chessboards and freaky weapons.

— G.H.