You will, no doubt, be familiar with the 1995 footage of a supposed alien autopsy. Since its yikes-inducing TV debut, the jittery black-and-white film is estimated to have been viewed by a billion people. Still, for better or worse, here it is again: a scrum of faceless hazmat suits hover over the corpse of a pot-bellied humanoid. Its forehead? Bulbous. Its expression? Pensioner outraged at price of bark chippings in local branch of Wickes. Over the next 18 minutes the suits proceed to dissect this appalled sod, slicing it open to reveal what appear to be various organs, condiments and splodgy, flopping … things.

“Those were lambs’ brains,” chuckles Trevor the butcher as The Alien Autopsy Scandal zooms in on a quivering hillock of the aforementioned horrors. Trevor was one of the individuals involved in the titular film, its production taking place not, as initially claimed, in a US military facility in 1947, but a Camden living room in 1995. Trevor had been approached by a sculptor to supply “guts” with which to stuff the “alien” mould that would, the latter had explained, be appearing in “a film”. Hmm. Nevertheless, guts – in the form of knees, hearts and miscellaneous entrails – were duly supplied. Anything else? “Pig eyes, ’cos they look like human eyes,” guffaws Trevor, before taking a hacksaw to the remains of a decapitated pig. Disgusting? Yes. But fascinating, too. And certainly no stranger than anything else in John Dower’s exquisitely directed documentary; a thing of great playfulness and eccentricity that, over three increasingly extraordinary episodes, unknots the tale behind the notorious film. Or at least does its best to do so. But the truth proves slippery and its gatekeepers are … well. Enter Ray Santilli (tinted glasses; deep shiftiness) and Gary Shoefield (tracksuit; air of one comfortable with the phrase “it is what it is”).

Close encounter … director John Dower. Photograph: Ryan McNamara/Sky UK Ltd

The London-based “music entrepreneurs” claim they bought a film in 1993 from a retired US military cameraman that captured the alien-strewn aftermath of the infamous 1947 “UFO” crash in Roswell, New Mexico.

On returning home, however, they discovered that the film had oxidised. Uh-oh. Not least because the friends had promised the footage to a US TV producer. What to do? Simple. Santilli and Shoefield would make their own version and pass it off as the original. Cue geysers of cash, jubilation from the-truth-is-out-there faithful and headlines of the “ET WE KNOW YOU’RE THERE!” variety. “A lot of people would call it a fraud,” says Santilli with an uneasy grin. “But it wasn’t.”

“To us,” says Shoefield, “it was a restoration of an existing work.” Sure, Gary. Anyway: media scepticism grew, the magnifying glass turned to the subject of government cover-ups and the retired US military cameraman became The Cameraman, a figure of near-mythical elusiveness and the only Earthly being who could confirm Santilli and Shoefield’s story. Who was this elderly chimera? Santilli’s still not telling.

The plot – and cast – thickens. There are incredulous ufologists, sweet-natured believers, still-furious-at-being-conned TV producers and a magician who refuses to appear on camera and is thus portrayed by a lip-syncing actor in a trilby.

There is an air of farce to proceedings, as Scooby-Doo baddies Santilli and Shoefield are pursued by the truth across the same endlessly looped stretch of cobblers and counter-cobblers. And yet. Several former US military personnel claim, with startling matter-of-factness, that there is indeed classified footage of aliens at Roswell. What’s more, they say, they saw this footage for themselves in the late 1970s. So, what really happened? And, in these darkening days of up-is-down-ism, does anyone really care? “People want to believe,” says one contributor, and I find myself pining for a time when a rubber dummy stuffed with sausages was greeted as a glimpse of heaven itself.