Richard John Bingham, the 7th Earl of Lucan, was declared dead in 1999, but that did nothing to halt the highly profitable cottage industry of speculation about him, which has chugged away since 1974, when he murdered Sandra Rivett, his children’s nanny, then disappeared. At first glance, you might think that this three-part documentary will be another contribution to this conspiracy-minded canon. But this is not a “have we found him?” film. I cannot emphasise enough how much it is worth sticking with it until the end. What unspools is a sometimes tender, sometimes troubling rollercoaster that ends up in surreal and unexpected territory. There is a fake monk, catfishing, drag queens and Timothy Leary. You have probably not seen this side of the Lord Lucan story before. The first episode is the most straightforward. The film-maker Colette Camden has found a new way of outlining what happened on 7 November 1974. A builder in Hampshire called Neil Berriman thinks he has tracked down Lucan, she explains. (There is a strong argument that the true subject of the episodes is Berriman, not Lucan.) Berriman’s mother, who had adopted him, would talk to him about a “brown envelope” containing information about his biological parents. For years, he wasn’t interested in even looking for it, but when he did eventually open it, it delivered a shock: newspaper cuttings about one of Britain’s most notorious crimes of the 20th century. It revealed that his birth mother was Rivett, the woman who had been looking after Lucan’s children for mere weeks when he bludgeoned her to death. As you would expect from a modern documentary, and one which involves Rivett’s son, this shifts its emphasis from the headline-generating exploits of the “fugitive Lord” – the profligate gambler and drunk, known to his friends, ironically, as “Lucky” – to the 29-year-old woman whom he murdered, the justice he eluded and the consequences of this violent crime for those left behind. Berriman has made it his mission to learn everything he can about the case – seemingly to the concern of his family – and episode one provides an overview of what he has discovered. There are interviews with people who knew Rivett, who knew Lucan, who attended the crime scene; and with people who, like Berriman, have made the investigation their primary focus (although without having the same personal attachment). It is here that the series starts to rev up and speed off into the distance, where it shape-shifts into something else entirely. Berriman has spent years working with the investigative reporter Glen Campbell, who has reported on the case extensively (and called his dog Lucan). They have pursued countless theories and potential sightings. The film joins them as they are on their way to confront a man they have tracked down in Australia, whom they seem certain is the aristocrat. To watch this with a critical eye is to notice that we, the viewers, cannot see or hear much of their evidence: a confidential police report that would compromise the job of the person who leaked it; a detail given off‑camera by Lucan’s brother. There are a lot of people saying “bullshit”. Often, there is no clear sense of what is true and what the people at the heart of this story want to believe. (There is, however, a clear sense of how much some of these men will make excuses for a friend who brutally murdered a woman – a small but deeply depressing detail.) It is not a rigorous investigation so much as an empathic portrait of human obsession. Camden is evidently fond of Berriman, and her involvement in the story grows more pronounced as the episodes progress. I kept thinking of The Journalist and the Murderer, Janet Malcolm’s classic study of journalistic ethics and the relationship between reporter and subject, wondering to what extent this documentary exists in a murky area. The programme gives Berriman (and, by extension, Rivett) a voice – and it’s hard to deny that he deserves that voice. By the time Lucan explodes into its surreal final act, you will be feeling astonished and uneasy about some of the people caught up in the whirlwind of pursuit. This extraordinary documentary lingers in the mind and leaves a lot more questions behind it than whether or not Lucan lived beyond 1974. Lucan airs on BBC Two and is available on BBC iPlayer