"Call me Mr Brown," the man said, as though he was in a conference call, not making a bomb threat.

Qantas flight 755 from Sydney to Hong Kong was carrying a explosive, he warned. And it was set to detonate as the plane came in to land.

If that sounds like a plot ripped straight from a kitschy Hollywood movie, that's probably because it is.

It's the larger-than-life tale of Australia's great plane robbery — one of the nation's most brazen aviation heists, born out of greed and undone by sheer stupidity.

What started with a chance rerun of the film Doomsday Flight inside a kitted-out van in Townsville some 48 years ago would inexorably set in motion a chain of events that ended in the extortion of half a million dollars — and stopped the nation in its tracks.

The bomb and the locker

It was May 26, 1971 — still some six months before the notorious criminal known as DB Cooper would enter the public consciousness after hijacking a Boeing 727 and parachuting to an uncertain fate — and an otherwise ordinary day in Sydney.

That is, until the phone rang.

The bomb was set to detonate if the plane dropped below a certain altitude. ( ABC News )

Mr Brown would tell staff at Qantas House he had hidden a bomb onboard an international flight to Hong Kong.

For a mere $500,000, he would lead authorities to its exact location, sparing the lives of all those on the flight.

But if you don't believe me, he taunted, why not inspect Locker 84 at Sydney's Kingsford-Smith International Airport where I've placed a replica device?

Mr Brown was not bluffing — or at least, that's what authorities were led to believe.

Inside the unassuming metal locker, police uncovered the unthinkable: a bomb constructed of gelignite with an altimeter-triggered detonator.

Captain William Selwyn said he did not know what altitude the bomb was set to go off at. ( ABC News )

With it was a note: should the plane descend below 20,000 feet (6,000 metres), the bomb will explode.

"If you don't pay, or if you interfere in any way, you will lose your plane. And this will be repeated," the letter continued.

Think the film Speed — but in the air.

"We were told to maintain our altitude at 35,000 feet," Captain William Selwyn would later say.

"[Because] we did not know what altitude it was set to go off at."

Authorities sprang into action. The replica bomb was defused, and the explosives were replaced with a light bulb.

There was only one way to test the veracity of Mr Brown's claims — and that was to take the duplicate to the skies.

The bomb was loaded onboard a second Boeing 707, and the plane climbed to 8,500 feet before beginning its precarious descent.

The replica bomb was placed in locker 84 at Sydney's international airport. ( ABC News )

When it dropped to 5,000 feet, the light bulb on the altitude activator came on — had the explosives remained inside, the aircraft would have been blown to smithereens.

This was not a game. Authorities had to act.

The Kombi and the pay-off

The 116 passengers on flight 755 to Hong Kong, blissfully unaware of the danger they were in, were told they were returning to Sydney because of a "technical fault".

In reality though all bets were off. Flight 755 was living on borrowed time.

Mr Brown demanded two suitcases full of cash, as demonstrated in this reconstruction outside Qantas House. ( News Ltd/Newspix )

The aircraft could never actually land lest Mr Brown's threats came to fruition, but it was slowly running out of fuel.

After hours of the plane circling the city, Qantas ceded.

Mr Brown, not one to be tested, would receive his ransom after all.

The money exchange was organised to take place outside Qantas House in Chifley Square, Sydney. ( National Library of Australia: Wolfgang Sievers )

At about 5:30pm, Qantas deputy general manager Phillip Howson took the call. It took less than 10 minutes for Mr Brown to detail the terms and conditions of the drop.

A yellow van would pull up outside Qantas House in Chifley Square in the city at 5:45pm. The driver would identify himself by shaking his keys out the window.

The getaway vehicle was not to be followed.

Any deviation from the plan would end in irreversible catastrophe, he warned.

Captain RJ Ritchie, a Qantas general manager, made the rendezvous to deliver the ransom as Mr Brown had instructed, and began pushing suitcases full of cash into a Volkswagen Kombi.

But there was a hitch in the operation. Four police vehicles parked in Chifley Square were never given the signal the drop was taking place — or so the rumour goes.

"Unfortunately, the plan didn't go the way in which it was designed," then-police commissioner Norman Allen would concede in the Sydney Morning Herald the following month.

"[The] van could not be kept under surveillance all the way to the spot when it was abandoned."

At 6:20pm, authorities received one final call.

"You can relax," Mr Brown said.

A newspaper clipping from 1971 describes the hoax and pay-off. ( Supplied: News Obscure )

"There is no bomb aboard the plane. You can land her safely."

And so, under the cover of nightfall, an incognito Mr Brown had fled with his earnings — leaving police none the wiser about the real identity of the criminal mastermind.

Who was Mr Brown?

Peter Macari was no stranger to the wrong side of the law, but no-one could have predicted the otherwise unremarkable man would assume the moniker that would spark a cross-continental investigation.

An English migrant, he had arrived in Australia some two years earlier on a false passport after skipping bail in Britain on an indecent assault charge.

Peter Macari was no stranger to the wrong side of the law. ( ABC News archives )

Macari's transition into Australian life, however, was far from smooth sailing.

After opening a small factory at Brookvale in Sydney which produced fibre-glass boats, he was reported to have lost half his life savings and began to travel.

It was on this jaunt across the country that his grand scheme was inexorably set into motion.

Inside what witnesses described as a "fitted-up van" where Macari had been residing in Townsville, the 1966 television-thriller film Doomsday Flight played on a small television set.

Set in the United States, the film sees a bomb threat made against a Douglas DC-8 airliner.

A bomb equipped with an altitude-sensitive switch is on board, police are told, and it will detonate if the plane tries to land.

Director Rod Serling would later lament having made the film — which authorities believe inspired three separate airline extortion plots — saying he had done a "vast disservice to airlines".

"I didn't realise there were that many kooks in the woodwork," he told the Nashua Telegraph newspaper some four days after the Qantas hoax.

"I wish … I had written a stagecoach drama starring John Wayne instead. I wish I'd never been born."

Regardless, Macari's plan had been set in motion. "That would be a good way to make money," witnesses would recall him saying upon watching the film.

A budding friendship with Francis Sorohan — who would later be charged and acquitted as a minor accomplice — sealed the deal.

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Sorohan sold Macari gelignite and detonators for a mere $100 during a trip out west, having stolen them from his employer, the Mount Isa Mines.

And so, all that was left to do was to make the call.

The manhunt and capture of Peter Macari

Catching the elusive Mr Brown was big business, and authorities vowed to leave no stone unturned.

Fifty thousand dollars was offered for any information leading to Mr Brown's capture. ( National Library of Australia: The Canberra Times )

Fifty thousand dollars was offered for any information leading to his capture, and detectives worked alongside Scotland Yard, Interpol and the FBI in a bid to narrow down the list of suspects.

Phonetic experts were brought in to listen to recordings of Mr Brown's voice, while sketches and flyers were released to the public — all, it would seem, to no avail.

Raymond Poynting's spending led to his capture. ( ABC News archives )

Ultimately, it was a tip-off from a service-station attendant about a "free spending" man — now known as Mr Brown's accomplice, Raymond James Poynting — that would undo the entire operation.

Though most would have the sense to lay low after pulling off one of Australia's most brazen heists, 28-year-old Poynting was not most men.

So when the former engineer-turned-barman — a regular customer — pulled in for petrol in a new E-Type Jaguar, it would come as no surprise that he turned heads.

When he returned some weeks later in yet another luxury car, those around him grew suspicious of his stories of good fortune.

Detectives of the Consorting Squad placed him under surveillance.

Poynting, proving there is no honour among thieves, confessed to his role in the robbery.

Police created replicas of the suspect, Peter Macari, in a bid to track him down. ( ABC News )

On August 4, 1971, Mr Brown and his co-accused were arrested.

Now all that remained to run its course was the epilogue.

The missing money — and comeuppance

Macari and Poynting were indicted in the Central Court of Petty Sessions in Sydney for their role in the hoax.

Prosecutors would allege Macari, or Mr Brown, was the mastermind behind the operation, while Poynting was charged as a co-conspirator, accused of aiding the operation.

Some of the ransom money was stashed inside a fireplace in Annandale. ( ABC News )

Both men would ultimately plead guilty, and the saga that had once gripped the nation drew to a close — or so it would seem.

Despite cracking the case, one question remained: where was the missing money?

A little over half of the ransom, some $261,387, had been recovered by detectives — hidden under floorboards in Balmain, a fireplace in Annandale and through the sale of a series of lavish cars — but the remainder had disappeared without so much as a trace.

Macari, true to form, spun authorities a tale of a wider criminal network that he had unwittingly been roped into.

A third man — the real mastermind, Macari claimed — had taken the lion's share of the ransom for himself.

"You said you gave $220,000 to a person known as Ken," Judge Staunton said upon sentencing, "this statement I reject."

It's a theory police too have unequivocally dismissed, although some still believe the missing money may be underwater off Bondi Beach, languishing in two safes.

Detectives count money recovered from properties across Sydney. ( ABC News )

Though the whereabouts of the ransom may remain a mystery, the fate of the men involved was sealed.

Poynting was sentenced to seven years in prison for his part in the hoax, while Macari was handed the maximum 15-year sentence.

The saga was finally brought to an ironic end on November 12, 1980. After serving nine years of his prison term, Mr Brown was deported back to Britain — on a Qantas flight.