Individually, they vigorously defend their own readings of the events of that Christmas. They condemn or defend Patsy, John or Burke, or all three. They wonder why the paedophilia angle wasn't better investigated. They scorn the three-page blackmail letter, alternately point to or dismiss JonBenét's bedwetting as evidence of abuse or as the hair-trigger motivation for Patsy to commit the crime. Green interviews mall Santas (involved in one of the more outré theories), and elicits a creepily impressive version of John Mark Carr, who falsely confessed to the murder, played by an actor whose level of research is a little frightening. One of the Johns sums it up neatly by saying that there is no theory that fits all the facts that doesn't sound crazy – though that said, the lady who hints heavily at Satanic involvement due to all the sixes (JonBenét was 6, and died on 26 December 1996) is maybe a little further off-piste than most. And throughout it all, Green intersperses fragments of scripted reconstruction, starring various Patsys and Johns in various configurations.

Those scenes, in which the actors finally act, are ramping up to a final section that is essentially an art installation. On a deliberately artificial soundstage, like a set for a sitcom, several rooms of the Ramsey house are recreated, connected by a hallway that leads off backstage. In them, a multitude of Johns, Patsys and Burkes act out, in the shared spaces, the various scenarios the film has described. This stunning, life-size, overpopulated diorama is simply a brilliant conceit, intricate and fascinating in what it says about the unknowability of truth and how the practice of mythologising is basically a Hydra of endlessly multiplying theories. JonBenét herself is the least present character here, and whether that is an appropriate omission – whether it feels like a respectfully empty seat at the table or a hole where the film's painful heart should be – is debatable. Casting JonBenét is many things: funny, provocative, clever, audacious, maybe even dazzling like a Vaseline-smeared, white-toothed pageant smile. But it is not sad, and perhaps it should be, just a little.

★★★★☆

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