Much like his spiritual mentor Nick Broomfield, Louis Theroux presents himself as a puzzled and lightly eccentric screen character. It’s a technique that disarms his subjects, allowing Theroux to dig and prod away, often with great success. The Church of Scientology, though, may be his biggest challenge. Naturally defensive, they are on the backfoot following Alex Gibney’s similar attempt at exposé, Going Clear; it takes all Theroux’s benign, befuddled English charm to crack this particular nut. And he has help, too, from defector Marty Rathbun, the pugnacious former enforcer at the organisation who is now Public Enemy No. 1, as far as the church and its leader, David Miscavige, are concerned.

Basing himself in LA, Theroux puts out a casting call for people to play Scientology followers, as well as Miscavige and Tom Cruise – the Church’s most famous adherent. Theroux’s strategy is to film ex-members’ stories using actors, replicating alleged practices never witnessed by outsiders. In some respects, his strategy echoes The Act Of Killing – Joshua Oppenheimer’s documentary in which the Indonesian killers of the Suharto era re-enacted their crimes on camera.

It is a cunning stunt on Theroux’s part, designed to draw the Church’s enforcers to him. Indeed, in an unexpected twist, it becomes clear that the Church is also making a film about Louis Theroux. This turn of events provides moments of pure slapstick – in one scene two camera crews face-off in a kind of Mexican stand-off. It seems, too, that the Scientologists have planted a mole among Theroux’s cast of actors. These moments of levity – as genuinely funny and diverting as they are – can’t quite disguise the fact that Theroux never really penetrates the Scientology movement as he claims he would like; but it does reiterate the sheer ghastliness of this particular cult.