Spoilers for “The Duke” ahead.

It’s the greatest heist you’ve never heard of.

The year was 1961. The United Kingdom was filled with pride that one of its national treasures — a priceless portrait of the Duke of Wellington by famed Spanish artist Francisco de Goya — was to securely remain in England after government funds were raised to stave off an interested American buyer.

The painting was proudly put on display in London’s National Gallery for all to view. But, just 19 days after it was installed in the museum, it was snatched in the early hours of the morning. 

John Bunton, a 20-something petty thief from Newcastle, was visiting the supposedly highly secured exhibition hall one day when he noticed that the window in the men’s bathroom opened just enough so that one could sneak in and out of it.

He left a matchstick on the window and tape on the lock to test if security would notice. When he returned the next day, nothing had been disturbed, so he hatched a plan. 

He got a gallery map and learned from a guard that the security system was turned off around 4 a.m., when the museum’s cleaning crew came in. 

John then made his way to a fenced-off lot outside of the bathroom window and waited until the early morning hour when the alarms would be disabled. He didn’t initially have a plan as to how he would get up to the window, but he swiped a ladder from a nearby builders’ yard. 

The heist

After climbing into the museum, John grabbed the painting from the easel on which it was displayed undetected and snuck back out the window and down the ladder. He then had to scale a tall barbed wire fence while carefully hanging on to the painting to get to his getaway car. 

Frantically driving off, he went the wrong way down a street and was pulled over. A policeman berated Bunton for his poor driving, but took no notice of his precious cargo. 

“He just waved him on to go the right direction,” said Christopher Bunton, John’s son and the film’s executive producer. “Everything just kind of fell into place, like God was helping.”

That was just the beginning of the epic heist, which lasted for years and was full of twists and turns. The events are the basis of the hotly anticipated new movie “The Duke,” which stars Jim Broadbent and Helen Mirren and hits theaters in April.

It’s “unlike any heist film you’ve ever seen,” Christopher told The Post.

Shortly after making his getaway, John realized that he had no idea what do with the painting. 

“So the first thing he did was call his dad Kempton [who] immediately insisted on taking the blame for it,” said Nicky Bentham, the film’s producer. 

Kempton had a heart of gold and a pet cause. He had spent the majority of his life in poverty and saw his father, a disabled World War I veteran, lose touch with society. As a result, he thought that senior citizens and those on hard times should get TV for free. 

“He believed television was the cure for loneliness,” said Christopher of his grandfather, who was jailed three times for refusing to pay licensing fees for the television in his northern English abode.

Kempton hid the prized painting in the last place authorities would think to look for it: a cupboard with a false door in his humble Newcastle home. Even his wife didn’t know it was there. 

No suspicions

“The government, Scotland Yard and the [National Gallery] were so shocked that this had happened under their nose and were really embarrassed by it,” Bentham said. “They were absolutely convinced that it had to be the mafia or some big criminal gang that could have done this because it had been so audacious.”

It so happened that the portrait had been stolen from the National Gallery exactly 50 years to the day after the Mona Lisa disappeared from the Louvre. Authorities thought maybe the two thefts were linked, the work of professional snatchers.

“The last person they thought it would be was someone like my grandad,” Christopher said. 

Kempton tried to use the painting to further his cause. Anonymously employing the Daily Mirror newspaper as a “middle man,” he sent them a shipping label from the portrait as proof he was in possession of the national treasure, and tried to get British authorities to pony up cash for his free TV idea. 

But, it didn’t work. In 1965, after four years, Kempton surrendered the painting by leaving it in a secured luggage locker at the Birmingham rail station. Five weeks later, he turned himself in to Scotland Yard, taking the fall for his son. 

The aftermath 

Kempton was defended by a famed lawyer named Jeremy Hutchinson (played by Matthew Goode), who found a loophole. At that point, the law stated it was not a crime to steal a painting if there was intent to return it, which was Kempton’s plan all along. He was found not guilty on all charges, save for stealing the painting’s frame, which had been tossed into the River Thames the night of the heist.

“When [Hutchinson] was interviewed years later, he said it was the most light-hearted, jovial court case he’d ever been involved in. In fact, he took it as a pro bono case even though he was one of the most famous lawyers in Britain at the time,” Christopher said of the trial, which saw onlookers pack into the courtroom.

“The only reason [Hutchinson] took it was because he thought it would be a laugh and a distraction from his personal troubles he was going through at the time. He said the whole courtroom was regularly in stitches from Kempton’s tales,” continued Christopher.

Years later, John confessed to stealing the painting when he was cuffed for a carjacking. But the Brits declined to prosecute, wanting to avoid reviving the public spectacle.

By way of a silver lining, Kempton’s overarching goal came to fruition in 2000, decades after his death in 1976. That year, laws were passed to award seniors over 75 with free TV.

How the secret got out

Though the heist was referenced in the 1962 James Bond classic “Dr. No,” with the portrait appearing in the titular villain’s lavish hideaway, the details around the Goya grab were shrouded in mystery for years. 

The heist was a family shame and had never been discussed while he was growing up — minus one time John drunkenly let the story slip out to his then-4-year-old son, Christopher.

Using his insider knowledge, the youngest Bunton combed through thousands of pages of documents put everything together and pitched it as a movie.  

“It was a really serendipitous moment when he got in touch with me and I immediately saw that it could be a brilliant film,” said producer Bentham, who signed on quickly. The two men knew they wanted Jim Broadbent to play Kempton. 

“He’s got that wonderful combination of warmth, a genuine tenderness and also a very natural humor and twinkle in his eye. We just had to have him,” Bentham said.

Mirren loved the story and also quickly signed on.

“It was really the script that she just loved,” said Bentham. “[And] the opportunity to work with Roger was always a draw. I think she also appreciated being sent something that was not necessarily an obvious fit for her.”

Christopher, who who has viewed the Goya on multiple occasions and says it “feels like family” thinks his grandfather would love how things turned out.

“If he was looking down now, he’d be very happy with the development of the film.”