Eight months after his relationship with not-quite-stepdaughter Soon-Yi was revealed in 1992, Woody Allen was accused of sexually abusing Dylan Farrow, his younger daughter with his partner Mia Farrow, when she was seven. Allen strongly denied all the allegations, as he has continued to do over the nearly 30 years since. But they fell on fertile soil. How could a man who could start an affair with his partner’s daughter not be capable of just about anything else, the argument went? A welter of publicity followed, claims (that no mother, especially one as devoted as Farrow, could possibly make such a thing up), counterclaims (that in fact she had done so, that Dylan was coached – which she denies, and that it was all an act of vengeance for the Soon-Yi affair), along with gossip and hearsay proliferated.

Dylan and her brother Ronan Farrow began speaking about the matter publicly themselves in 2014. This has disinterred old battle lines and occasioned new accounts from people who were close to the family at the time and from Moses, another of the Farrow children, which contradict either things they said at the time or the saintly mother narrative that seemed the most natural one. Meanwhile, doctors examined Dylan at the time and found no evidence to support Farrow’s contention, and Allen was investigated by the Yale New Haven hospital’s child sexual abuse clinic and the New York State’s social services department, who reached similar conclusions.

Journalism’s job, the truism goes, is not to hear from each side what they think the weather’s like but to stick your head out of the window and see if it’s raining. The new HBO documentary Allen v Farrow (Sky Documentaries) has a running length of four hours, which would be ample time to do so. Alas, though, the film attempts to be an investigation, we do not hear from both sides and are working with the curtains very firmly shut. It is the Farrows’ show throughout. Dylan, Ronan and, above, all Mia lay out their version of events without any interruption other than footage of home videos, interior shots of the beautiful Connecticut home at which the ugliest of events is alleged to have happened and selected clips of Allen in chatshow interviews going back decades. Occasionally, we hear excerpts read out from his autobiography published last year, but it’s hard to feel that this constitutes a proper response given Allen declined to take part.

Which makes it what? A four-hour PR piece, at one level, another front opening up in the continuing battle to establish which asserted truth wins public hearts and minds – all the victory, in the absence of a trial, that is left. At another level, though it doesn’t ape the style of the true crime genre that has flourished since Making a Murderer became such a hit for Netflix, it feeds those same appetites. It leaves itself open to the same charges of pandering to an audience’s worst appetites and exploiting some or all of those who take part. And while the first-tier contributors to this genre reinvestigate, adduce new evidence and/or bring to light miscarriages of justice, Allen v Farrow is in the second class – which simply re-expose and pick over the bones of what went before.

I am not passing comment on the rights and wrongs or facts or fictions of the situation – I have, after all, like all but a small handful of people in the world, no idea of the truth. But that’s really the point. If you are going to present four hours of information on a contentious subject, you need to add value to it, stress-test it, go deeper, not simply regurgitate what one party has told you – however much sympathy you feel for them, however much you believe them. Especially then, in fact, because a wholly uncritical, uninterrogated presentation eventually allows the suspicion that the story couldn’t withstand it.

In the first episode you might point as an example of this to the moment when Mia Farrow lists how much predatory behaviour she claims she saw from Allen, and yet is not pressed on why then she did not act as we might expect. And so another layer of doubt and obfuscation accretes around a story and around people who – on both sides and whatever the facts of the matter actually are – deserve either clarity or to be left alone.