Cinemas that showed movies continuously, often around the clock, were called “grindhouses,” and back in the ’70s, these were the cinematic epitome of urban decay, moral drift and “alternative” movies.

It’s an American term most often associated with New York movie houses where one could watch Blaxploitation films, B-movies and the like, or just buy a ticket to get in from the cold or heat and nap through movie if it didn’t hold your interest.

But in Britain, there was once a grindhouse that transcended the label, an alternative cinema where classics, “out there” horror, gay films, porn and most any type of movie that had a following would be shown. At The Scala, cult films and their cultist fans could gather in a cult grindhouse of worldwide notoriety.

Of course John Waters was their patron saint.

“Scala!!! or, The Incredibly Strange Rise and Fall of the World’s Wildest Cinema and How It Influenced a Mixed-up Generation of Weirdos and Misfits” is a messy, amusing and misshapen history of a theater that long predated its most notorious era — 1978-1993.

A portrait emerges of the place, the time and the people it served is created by interviewing scores of fans, filmmakers inspired to make movies, musicians who joined bands, comics who took to standing up at a mike and gays who decided to “come out” thanks to the decadent free-for-all that was this glorious and ancient movie house, sometimes music venue and social magnet for punks and anybody else who felt out of place in Margaret Thatcher’s Britain.

Ralph Brown, who played a punk’s punk in the quintessential big screen goof on that era, “Withnail & I,” shows up and suggests the Scala was the best place in Britain to find a lot of girls and boys like the characters in that movie, especially his — Danny.

Cinephiles, William Castle cultists, Derek Jarman worshippers, aspiring artists and filmmakers, teens misspending their youth and “just people who didn’t want to go to bed on Saturday night” haunted this cinema, which operated in a couple of locations — and hosted IggyPop, Lou Reed and Bowie shows at one time –before settling in seedy Kings Cross where it was meant to be all along.

Fans eagerly snapped up the colorful, forbidden fruit-laden monthly program calender, with its promise of “Eraserhead,” “Pink Flamingos,” “The Evil Dead,” “W.R.: Mysteries of the Organism,” its programmed celebrations of the Brit TV series “The Avengers” and “Shock Around the Clock” film festivals.

Drug abuse in the ticket line, cats wandering the auditorium, sex in the toilets, suicide out the windows — The Scala had it all, a venue where membership was required, “like joining a secret club,” “the Sodom Odeon” of British moviegoing.

It was a scene, man.

Waters leads the cast of interviewees, a delightfully deviant cheerleader who showed his films there, took friends and cast-members from his movies to experience the place and sets the tone here, reminding us of the importance in helping one find one’s tribe.

“The cinema was an important as the movies,” one interview subject notes, a big, dark old movie palace “that looked like an abandoned embassy.” Like such theaters in New York, and their later imitations (the indie Angelika Film Center), the subway was downstairs, giving the movies an aural rumble that wasn’t on the soundtrack.

For a film buff, “Scala!!!,” co-directed by a former programmer/manager Jane Giles, who wrote a book about the place, is a cinematic flash card, with every title — “King Kong” (the original) to “Koyaanisqatsi” to every kung fu movie of the “Bruce Lee LIVES!” era — inspiring memories of Russ Meyer smut, Waters before “Hairspray!” became a musical and inner city movie theaters before home video ruined the communal love-in that this sort of movie-going experience could be.

I got a taste of New York at the tail end of the grindhouse era. And many a big city indie cinema has at least dabbled in the Scala-styled program-for-the-aficionado, the “alt lifestyle” and just plain “weirdo” corners of film fandom.

But even if you missed all that, “Scala!!!” should amuse and confuse and titilate, providing a history lesson that reminds us that every time the culture tries to turn conservative, the fringe dwellers find the like-minded and strike back. There’s nothing more “punk rock” than diving into a movie some people warn you that you must never see with a whole bunch of like-minded free thinkers.

Rating: unrated, clips of films featuring nudity, sex and violence, with profanity, drug content

Cast: John Waters, Beeban Kidron, Mary Harron, Adam Buxton, John Akomfreh, Ralph Brown and Jane Giles

Credits: Directed by Ali Catterall and Jane Giles. A Severin Films release.

Running time: 1:36