Over the past few months, a strange story has been seeping into the mainstream media from the more excitable corners of Substack and YouTube. Its claim: scientists whose work related to aerospace and nuclear research are either dying or going missing. According to an influential report in the Daily Mail in March, the disappearances form a “chilling pattern”: two, for instance, had worked together at an air force laboratory. The implications, in some accounts, are Hollywood sinister, with scientists working on top-secret breakthroughs running into dark forces who wanted to get hold of what they knew – or ensure their silence. And it all seems to have something to do with what we used to call UFOs.

On examination, these claims collapse. The “scientists” actually worked in disparate fields, from chemical biology to plasma physics. Several were actually administrators. Two had retired. One died of natural causes; another in a shooting spree. In any case, as the debunker Mick West pointed out, the “US top secret-cleared aerospace and nuclear workforce” is around 700,000, so normal mortality rates would predict far more deaths over the 22 months concerned – about 4,000. Nonetheless, Congresspeople have been warning darkly of threats to “national security”. The Trump administration has launched an investigation into a phenomenon that is often said to go hand-in-hand with something called “Alternative 3” – whose origins might end up surprising Trump and co.

On 20 June 1977, an edition of Anglia Television’s Science Report was broadcast on ITV. It set out to investigate the “brain drain” of British scientists to the US. But it emerged that some of these scientists had vanished completely, while others had died in strange circumstances. The journalists had stumbled on something huge. As the host, former ITV newscaster Tim Brinton, solemnly explained, the greenhouse effect would soon make the Earth uninhabitable, and this had forced the powerful to choose between terrible alternative solutions. The American and Soviet governments had decided to work together in secret to implement “Alternative 3”: building a launch base on the moon, and from there a “human survival colony” for the elite on Mars. The missing scientists had been co-opted to play their part; the dead ones had threatened to leak the plan.

As you probably guessed, the “documentary” was a drama – as signalled by end credits listing the actors who played the horrified reporters and terrified scientists. Science Report did not exist; the whole thing was invented by a screenwriter named David Ambrose. He had been trying to write about people going missing, when he hit on the idea of a mock-documentary about people disappearing to Mars, driven to flee Earth by a novel concern: pollution-induced global warming. From there, he tells me, the script “wrote itself”, drawing on the breathless tropes of onscreen investigative journalism in the age of Watergate: secretly recorded street interviews, hidden truths on poorly-recorded tapes, the terrified witness who knows too much.

The story keyed into serious anxieties about the future, but the idea of playing with these in a pretend documentary raised alarms. “Everybody in Anglia Television was wetting themselves with fear,” Ambrose remembers. But Anglia’s founding executive director was one of the great British movie moguls, Sir John Woolf, who’d produced the likes of Ava Gardner and Humphrey Bogart. And he “just loved it, because he knew exactly what the impact would be, and he just overruled everybody and said: ‘Go ahead!’”

To give the show gravitas, they approached Brinton, who was warned by friends not to get involved, given that he was trying to become a Conservative MP. Brinton ignored them, played the earnest anchorman absolutely straight – and won his election regardless. Brian Eno was commissioned to write an eerie piece of music. The production designer Terry Ackland-Snow, who had worked on Stanley Kubrick’s 2001: A Space Odyssey, ingeniously evoked signs of life under the Martian landscape by dragging a nail under sand. They acquired footage from Nasa and added sensational fake astronaut voiceovers – a mix of fact and fiction that Ambrose wryly concedes was “completely nefarious”.

The show was meant to be broadcast on April Fools’ Day, but fatefully, it had to be moved back, and went out on 20 June. Ambrose says he “intended to cause a flap” – and he did. While those end credits did begin with a dateline saying “April 1st”, many people took the show seriously. ITV was hit by a barrage of calls from viewers – some protesting, others seeking reassurance the programme was fiction. The Scottish Daily Record headlined the row “TV TERROR!” Ackland-Snow had an incensed Jehovah’s Witness knock on his door to tell him he should be ashamed of himself.

Alternative 3 was broadcast simultaneously in Canada, Iceland, New Zealand and Australia – but not in the US. ABC wanted to network it, but was forbidden by broadcasting rules. It was initially visible only to those Americans whose TVs picked up Canadian programming. However, 1978 brought a spin-off book. Ambrose was too busy to write it, but with his blessing, a journalist called Leslie Watkins was brought in. Watkins wove in more 1970s nightmares – drawing on revelations about CIA attempts at brainwashing to suggest that Alternative 3 involved “adjusting” batches of humans to turn them into slaves. It also suggested that the claim the documentary was a hoax was a cover story.

And so Ambrose’s fiction escaped its British origins to take up residence in the strange dreamscape of American conspiracy theory. As the political scientist Michael Barkun traces in A Culture of Conspiracy, the show’s notion that the elite was plotting to abandon Earth keyed into existing visions of imminent apocalypse. Evangelical Christians’ belief in the Rapture, for example – when the chosen few, supposedly, will vanish, leaving everyone else to their fate.

Alternative 3’s afterlife really took off in 1991, when the conspiracist Milton William Cooper included it in his book Behold a Pale Horse. The book’s paranoid tales of secret government evil, “evidenced” by fictions like Alternative 3, influenced not just paid-up conspiracy theorists such as Alex Jones, but everything from the UFO-obsessed, nothing-is-as-it-seems world of The X-Files to an array of hip-hop stars. On Nas’s 2008 track Testify, for instance, he name-checks “William Cooper, who told you the pale horse is the future”.

Cooper fused Alternative 3 with theories about Aids, depopulation and the Kennedy assassination, while insisting that Science Report was a real series. Another influential theorist, Jim Keith, wrote Casebook on Alternative 3, complete with a chapter on “Missing Scientists”. The book begins by acknowledging the story appears to be a hoax. But what if that claim is just an elite trick? Meanwhile, other conspiracists were worrying about who would get to go to Mars: just how senior a Freemason did you need to be? Ambrose himself was visited out of the blue by a young man.

“He said he’d come all the way from California,” the writer recalls, “and he just really needed it to come to the horse’s mouth. He seemed perfectly sane, perfectly nice. We chatted. I said, ‘No, I’m terribly sorry. I did make the whole thing up.’ And he was very crestfallen.”

How does he feel about the fact his 50-year-old conceit still has some people convinced today? “Frankly: ‘Wow, it really worked!’ A classic of its kind.” The question of what people are willing to believe, he considers, is itself “something worth establishing as a theme”. He cites an old saying: “it’s easier to fool someone than to persuade them they’ve been fooled.”

Alternative 3 shows just how much impact a well-constructed spoof can have, even when its creators protest that it’s not real. It’s far from the only one. Orson Welles’s 1938 radio version of The War of the Worlds famously caused much anxiety with its “news” bulletins about the alien invasion of New Jersey. (Ambrose worked with Welles as a young man, but says War of the Worlds didn’t influence Alternative 3.) The most strikingly similar example, however, is Report from Iron Mountain – a 1960s US government report warning that world peace would destroy the US, which was concocted by anti-war satirists. Like Alternative 3, this has long been taken far too seriously by conspiracy theorists, who’ve used it to back up their fears: that the government is planning to enslave the population, and that environmental calamity is a cover for imposing tyranny.

The basic nightmare here – that the government is infinitely powerful and infinitely evil – also lurks beneath today’s “missing scientists” flap. The real nightmare is that today much of the US government is itself seen as being run by conspiracy theorists.

So – looking back from today’s world of deepfakes and disinformation, doesn’t concocting a hoax documentary “exposing” an evil government plot seem a bit dangerous? Ambrose says the situation in 1977 was “very different” – and concocting a mockumentary seemed radical. “It was a young man’s thing. You’re not scared, you’re just more keen to make an impact.” He was really just following the logic of his idea.

“I was trying to write a play,” he says. “Funnily enough, I never, ever thought of it as a hoax.”