Inside a bank vault, collecting dust somewhere in Manhattan, lies a fascinating pop-culture curiosity from the 1980s.

It’s the sound recording from a B-movie called “Grizzly 2.”

You’ve probably never heard of the film. You’ve certainly never seen it. But the story behind it is bizarre, even by Hollywood standards where bizarre is the norm, and the tale involves — among other mishaps — stolen money, malfunctioning special effects and a script that was rewritten by none other than its Hungarian caterer.

It also holds the distinction of being quite possibly the only time in history when two Academy Award winners have starred opposite a giant, mutated bear.

And that’s probably why anyone still cares about “Grizzly 2” some 30 years after the movie was originally shot but never released.

As the title suggests, “Grizzly 2” was a sequel to “Grizzly,” a low-budget 1976 horror movie about a bear terrorizing a state park. It was basically “Jaws,” but, you know, on land. With a bear.

It was directed by someone named William Girdler, who three years earlier had helmed “Three on a Meathook.” Despite its lack of pedigree, “Grizzly” became a financial winner.

“It was a huge success, earning 50 times the cost of production and was the top-grossing indie film of the year,” says David Sheldon, the movie’s screenwriter/executive producer.

“Grizzly” raked in $39 million, prompting producer Ed Montoro to approach Sheldon to ask about a sequel. Sheldon and wife Joan McCall penned a script, but Montoro couldn’t raise the funds.

That’s when another producer, Joseph Proctor, stepped in. Sheldon agreed to move forward on the condition he could direct.

The movie’s plot was pretty straightforward: Music lovers gather in a national park for a massive, Woodstock-like concert. Nearby, a giant, 20-foot-tall grizzly goes on a murderous rampage after poachers kill its young. As the bear threatens the concert, disemboweling people along the way, the park’s superintendent (played by Louise Fletcher, who won an Oscar for “One Flew Over the Cuckoo’s Nest”) is determined that the show must go on.

A French hunter (John Rhys-Davies from “Raiders of the Lost Ark”) is sent to kill the bear, but the animal makes it to the concert anyway. It’s finally electrocuted on stage and killed.

But what’s most remarkable about “Grizzly 2” is who played the teenage victims: Laura Dern, Charlie Sheen and George Clooney, in his big-screen debut. The trio played campers who are killed by the monster. Dern and Clooney — in classic horror tradition — have a love scene before they meet their bloody end.

“Clooney was charming and had charisma,” Sheldon recalls. “Sheen was handsome, was the son of Martin Sheen, who we knew, . . . and it looked like he was going to make it big. Dern was simply terrific and the daughter of Bruce Dern.”

Laura Dern remembers it well.

“I mean, I’m 16 years old, it’s six weeks in Budapest, Hungary . . . and it’s me, George Clooney, and Charlie Sheen,” Dern told The A.V. Club in 2011. “I’m not gonna say another damned thing. Except that it was the craziest time. And the paprika chicken was outstanding.”

Not all the Derns were happy about Laura’s role, Proctor claims.

“Her father, Bruce Dern, threatened to kill me over the part . . . LOL,” Proctor said in an email.

(Through his agent, Bruce Dern said, “This is totally not true” — though Dern had to be reminded his daughter was even in “Grizzly 2.”)

If there was tension, it might have arisen from Dern’s love scene with Clooney.

“I heard from one of the producers that [Laura] had been asked to do a nude scene and refused,” Sheldon said. “They then told her that they would use a double to do the nudity.”

Total screen time for Dern, Clooney and Sheen is about five minutes. And yet, incredibly, Sheen turned down the lead in “The Karate Kid” to take this role on the advice of his father. Charlie lamented the decision on his 2011 “Torpedo of Truth” tour.

One of the bigger challenges of the 1983 production involved staging the concert. The filmmakers decided it should be shot overseas, so Proctor partnered with Suzanne C. Nagy, a Hungarian economist-turned-producer.

“This movie would never be able to get a permit in the United States,” says Nagy, now an artist and gallery owner living in the West Village. “To make a huge concert on film in a rural area, it was almost impossible. Getting all the environmental permits, etc.”

So the production headed for Hungary. Only no one had told its supposed director, Sheldon.

“The cast and crew were flown to Hungary without our knowledge and we were left behind with no explanation,” Sheldon says. “They had engaged a Hungarian director who had never directed a movie before. See what’s coming?”

But no one could see what was coming.

Director André Szöts (who died in 2006 with “Grizzly 2” as his only directing credit) and the crew secured a Russian army base that was to serve as the concert venue. Instead of faking it, the filmmakers hired a Hungarian promoter to throw an actual concert.

Some 50,000 fans showed up for the three-day event, paying around $15 per ticket, which was a lot of money in Hungary at the time. The middling bands came from Hungary, England and America, and included Toto Coelo, the forgettable New Wave group behind “I Eat Cannibals Part 1.”

Filming began, with one hitch: Some scenes needed to be rewritten, so the producers naturally turned to the most obvious choice: the movie’s caterer.

“The caterer rewrote the scenes, then started monkeying with the rest of it,” says original script writer McCall. “The result was so disjointed that they couldn’t cut the film together.”

That was hardly the only problem. Instead of using real bears, the production hired veteran effects man Nick Maley to create animatronic ones. Close-ups were to be shot with a man wearing a bear suit.

“The bears seriously did not work,” says film editor Ross Massbaum.

As a result, the bear movie had no bear. All of the principal photography was eventually completed — without any shots of the grizzly.

Along the way, there was also the minor issue of much of the movie’s funds disappearing, which halted production until Nagy could raise more money.

Where did the money go? More than a few fingers point to producer Proctor, the most shadowy character of the saga.

“He disappeared on me,” Nagy says. “He took the entire fund, if I remember correctly. I had to go to Chicago to testify, because I had to prove that I was not part of it. He got a minimum of $2 million that he later took and disappeared from this country. The guy was nowhere to be found.”

Proctor, however, is much easier to find these days. He’s locked up in a Los Angeles federal prison, inmate #57751-112, serving five years for tax evasion.

He distanced himself from the “Grizzly 2” theft in an email sent from prison.

“As for the money, only two people had access to all the funds,” Proctor wrote. “The investor and his accounting representative.”

Proctor says the movie was never finished because the investor ran out of cash and the artificial bears never worked.

“Once I am resentenced I will be out and I can give you the true story, which is incredible,” he wrote.

Don’t hold your breath. It’s unclear if anything Proctor says can be trusted. In addition to failing to report $950,500 in income, for which he was sentenced to five years in 2012, Proctor, 68, was also accused by his former employer, PowerHouse Studios, of diverting “significant funds” from the company. And he was also rumored to have pulled a disappearing act similar to “Grizzly 2” on the 1980 Jerry Lewis movie “Hardly Working.”

Today, the footage continues to sit in storage at a Paris photo lab, as it has for decades, with the soundtrack in a separate New York vault. Nagy owns the rights and says she’d consider selling to someone who wanted to finish the film — as long as she doesn’t have to be involved. She long ago left the movie biz.

Co-producer Massbaum is more gung-ho. He approached Clooney’s agent earlier this year with a proposal for the actor to shoot new footage and finally — finally — release the film.

“That proposal received considerable interest at [Clooney’s agency CAA] based on the actor’s expressed interest not to leave behind a picture made early in his distinguished career,” Massbaum claimed.

So it’s a go? Not so fast. A rep for Clooney said the actor has no plans to get involved with “Grizzly 2” again.

Horror fans probably have to face the reality that the movie will never be released. And that’s OK. The story behind “Grizzly 2” is probably better than the movie itself.