Unless you’re particularly interested in cults – and French cults at that – there’s a strong possibility that you’ve yet to come across the Order of the Solar Temple. For the uninitiated, the OTS was a religious cult active from the mid-1980s to the mid-90s, basing its teachings on a grab bag that included new age fluff, Freemasonry and ideas pilfered from the Knights Templar. Its founders – jeweller turned spiritual guru Joseph Di Mambro and a charismatic homeopath named Luc Jouret – succeeded in recruiting several hundred members across Switzerland, France, Canada and even a handful on the French Caribbean island of Martinique.

In its promo videos, the OTS steered away from bells and smells to promote its ecological credentials, with members living off the land in its technicolour communes. But there was a catch. Like many cults before and since, the Order used its members for free labour and to gain access to their life savings, promising them eternal life if – and only if – they took their own lives in order to ascend to a planet called Sirius. More chilling still, the sect simply murdered many of those who didn’t kill themselves voluntarily. In total, 74 followers died between 1994 and 1997, including Jouret and Di Mambro themselves.

And so to Sirius: An Apocalyptic Order, a four-part series soundtracked by pulsing French electro, with some of the snazziest graphics ever committed to primetime BBC Four. It also offers an almost overwhelming amount of information on the media frenzy that exploded around the OTS in the French-speaking world, where it gained the same ubiquity as the likes of Heaven’s Gate or the Waco massacre across the Atlantic.

First broadcast in France in 2022, the series features testimonies from former cult members and the devastated relatives of the deceased. However, the main focus is the camaraderie that developed between three journalists who investigated the OTS, and would go on to co-author a book about their efforts. Our crack team are Bernard Nicolas and Gilles Bouleau from France, and Arnaud Bédat from Switzerland.

While Nicolas and Bouleau are in camera-ready reporter mode as they recount various horrors, Bédat is brilliantly candid and off-the-cuff (I really warmed to him while watching, and was sad to discover he died last year). While his fellow journalists were somewhat new to cults, by the time the OTS appeared Bédat already had a reputation in his office as “Monsieur Secte”, he says, and he frequently sums up the case with lines that are as droll as a talking head in a documentary about a murderous cult can be (at one point, he contrasts the reality of the leaders living it up with their stolen cash with the image of followers condemned to “breaking stones and growing lettuce”).

Much of the detail here is, of course, far from amusing, especially in the opening double bill. The series begins in Cheiry in Switzerland, where a fire at a farmhouse leads police to a crypt where 23 bodies are found with fatal gunshot wounds. Meanwhile, 100km away in the village of Salvan, another blaze was raging, again leading authorities to a room packed full of corpses. The series often flits between such events, emphasising the spread of the OTS and the influence wielded by Di Mambro and Jouret, who is resurrected for viewers via video footage of his proselytising. However, moving between these copycat cases doesn’t make it the easiest story to follow – not least when our journalist trio is duly dispatched to investigate more deaths 4,000 miles away in Quebec (while this was to become an important chapter in the case, no one was particularly bothered at the time, Bouleau notes – so much so that his bosses didn’t even send a camera operator).

As the series progresses and the suicides continue, the disturbing details pile up, including Di Mambro’s belief that his infant daughter – a mascot of sorts for the OTS – was the Messiah. Slowly, the programme’s focus shifts to Michel Tabachnik, a prominent Swiss conductor whose former partner took her own life while in the cult. Despite his protestations, the journalists and (crucially) prosecutors suspected Tabachnik may have had more involvement with Di Mambro and Jouret than he was letting on – claims he continues to deny into the final episode. By this point, you might get the sense that the makers of this series have buried the lede a little. Even so, if you are truly interested in cults, and French cults to boot, then Sirius: An Apocalyptic Order could well keep you hooked. For anyone else, it may feel too much like seeing how the true-crime sausage is made.