“True crime” documentaries usually stick to a formula — depict the crime, then show the investigation and if there was a solution, how the investigation came out.

But what if you’re not absolutely certain there was a crime? There is no physical evidence, no murder victim’s body, no dead-certain suspect and no motive?

Try making a film out of that. Better still, try building a case “Out of Thin Air.”

That’s just what is depicted in the British-produced documentary of that title, a tale of young people convicted of murders based on a single piece of the evidence puzzle — confessions.

No, this isn’t another movie about the “West Memphis Three.” It’s set in monocultural, ethnically pure and thinly-populated Iceland just a couple of years after it had the world’s attention by hosting the Fischer/Spassky chess match. And perhaps the one object lesson the story the film tells has for other Western democracies is, “if this could happen there, it could happen anywhere.”

In 1974, a young man doesn’t come home from a night of bar hopping and partying. It’s the dead of Icelandic winter, but search parties head out into the frozen lavascape that is this remote and forbidding island, searching in vain for some sign of him.

Did Guðmundur pass out and tumble into a crack in the Earth, fall into the sea? Or did something more sinister happen?

The police focused on where he went, who last saw him, as we’d expect. They can find nobody with a motive for the crime, even though there’s this one suspicious fellow with an Eastern European name, Sæv­ar Marinó Ciesi­elski, who gets their attention.

Nothing comes of that.

Six months later, winter’s back, and an older man, a father, Geirfinnur, disappears even more mysteriously. Vast search parties, more poking around in snow and lava fields, on beaches — nothing. But they remember this foreign guy, and people who partied with him. They’re especially interested in  Erla Bolla­dótt­ir, his Icelandic girlfriend, pregnant with his child.

Months of investigating, endless interrogations of those two, and others who knew them, the revelation that Erla and Sæv­ar had been defrauding the phone company out of large sums of cash, and the cops are sure they have their quarries. Because outside of the investigation, the case is being tried in the equally insular world of Icelandic media. Leaks, revelations, the whole island — where everybody is related — is sure this “gang” did it.

Exhaustive investigations are replaced with exhaustive trials. Still no bodies, evidence of crimes, murder weapons or motives. But if you hold the floor in court long enough…

Now, forty years later, people are finally having their doubts.

Dylan Howitt’s film recreates the “crimes,” or recreates the police recreations of the crimes. He interviews cops, journalists, a memory expert and those who knew the missing men as well as the survivors among the six people accused and convicted of their murder.

“Out of Thin Air” is on its most solid ground pounding home the notion that “memory,” as Erla says, “is such a” fragile, strange thing. It can be manipulated, tricked and twisted by those determined to do it.

Interrogate somebody 180 times, for hundreds of hours, park them in solitary confinement for days and weeks on end to “concentrate” and try to remember details you’re suggesting to them, they just might confess to whatever you put in front of them.

“Out of Thin Air” cannot quite summon up the gossipy atmosphere — alleging governmental involvement, conspiracy and cover-up — the “public hysteria” for a resolution to these cases in what is “kind of a hobbit society.”

Frustratingly, Howitt makes little attempt to recreate the lives of the victims or re-investigate the disappearances. The movie feels incomplete, as indeed the police case still does.

Instead, he focuses on the jaw-dropping case coerced and constructed out of arrests, releases, re-arrests and years of interrogations and incarceration, turning lover against lover, using this coerced conviction to keep people in jail while that coerced conviction is trumped up and added to it.

It sounds like justice in China, not a Western European democracy. And it literally could happen anywhere in which blind justice is worse than blind, and a compliant public believes what they’re told to believe instead of what common sense is putting right in front of their face.

Sometimes, that coup de grace in any case, the fixture of many a police procedural and boilerplate courtroom drama, the “confession,” is the most worthless evidence of all.

MPAA Rating: unrated, descriptions of murder, drug abuse

Credits:Directed by Dylan Howitt. A Netflix release.

Running time: 1:24