The very title “My Scientology Movie” suggests a juvenile goof-off on a serious subject. It could even be read as a dryly self-deprecating acknowledgment — as befits writer-presenter Louis Theroux’s very British demeanor — of its shagginess in the wake of Alex Gibney’s searing Scientology inquiry “Going Clear” earlier this year. Underestimate Theroux at your peril, however: His and director John Dower’s approach may be a lot daffier than Gibney’s, complete with casting sessions for a fake Tom Cruise and an uninvited cameo from a bikini-clad starlet, but this riotously funny doc yields its own penetrating insights into the fiercely guarded administration of the church that Ron built. It’s also a witty essay on the politics of surveillance, as it emerges that Theroux is in turn being filmed by those he’s investigating — a twist as anxiety-inducing as it is absurd. Ancillary returns, given a still-hot topic and Theroux’s substantial following, should impress the pic’s auditors.

The possessive adjective in “My Scientology Movie” is perhaps misleading. Theroux — the lanky, likable broadcaster and docmaker best known for his globe-trotting BBC specials — may be front and center throughout this characteristically good-humored, personality-driven project, but it’s another man who turns out to be its conflicted star: Mark “Marty” Rathbun, a former senior executive of the Church of Scientology who, after 27 years in the institution, acrimoniously departed in 2004. As one of the Church’s most outspoken opponents, Rathbun was also prominently featured in “Going Clear,” but Dower and Theroux use him here as far more than a talking head — inviting him to co-direct vividly imagined re-enactments of behind-the-wall activity, including the allegedly violent scare tactics of Church leader David Miscavige.

Yet collaboration comes with its own own thorny complications, as Theroux can’t resist pondering the ex-member’s degree of complicity in the “terroristic” activity he now condemns — notably the rumored abuse and incarceration of perceived dissidents in a confined area (known as “the hole”) of the Church’s high-security Gold Base estate in rural California. Rathbun bristles hotly at insinuations of his own involvement, but Theroux’s needling of his notional ally isn’t just cheeky journalistic provocation. Rather, his unwelcome questioning of Rathbun’s past serves to remind viewers that we may or may not be dealing with an unreliable narrator, one dismissed by the Church as “an embittered liar with (an) ax to grind.” The institution’s word, of course, is even less transparent than his; the pic’s question isn’t so much whom to believe as whom to let steer our imagination.

Theroux finds similarly disgruntled (and similarly Church-discredited) former Scientologists to indirectly corroborate Rathbun’s compelling testimony — though Jeff Hawkins, a former member of their elite, militaristic Sea Org wing, suggests he’s withholding key information. Needless to say, no current Scientologists have come forward to present their side of the story, barring involuntary on-camera appearances from senior official (and Hawkins’ ex-wife) Catherine Fraser and a bizarre coterie of unidentified “squirrel busters,” recruited to menace Theroux and Rathbun into desisting.

Holding true to their favored method of seeking truth in farcical illusion, then, Theroux and Dower compensate for that absence through masquerade. Enlisting a non-affiliated actor to play Miscavige according to Rathbun’s script, they stage detailed dramatizations of supposed goings-on at Gold Base. These mini-plays can’t claim any certain basis in fact, but even at a hypothetical level, they’re riveting — not least because the team’s chosen thesp, Andrew Perez, is so chillingly persuasive as the religion’s externally glib but possibly paranoid leader. Astonishing performances in documentary re-enactments are rare indeed, but Perez barrels in as a man possessed, seemingly channeling several “Glengarry Glen Ross” characters at once and lending gravity to what could have been a shtick-y exercise. (A Wayfarer-wearing Tom Cruise stand-in, by comparison, barely registers.)

Any viewers after an extension of Gibney’s tough-minded research project may be bemused by Theroux’s more playful brand of speculative satire and outright burlesque — complete with a thrumming, exaggerated orchestral score by Dan Jones. As Theroux defies signage to explore the perimeter of Gold Base and rankle Church guardians along the way, this is still shoe-leather reporting of a sort, but the shoes in question are closer to flip-flops. Either way, it’s certainly enough to concern Scientology brass, who begin tailing Theroux’s shoot with camera crews of their own — a reversal of the lens that a fictional conspiracy thriller could hardly have executed with more elegant irony. (In one hilarious scene, an iPhone-wielding Theroux and a Church-employed cameraman circle each other silently in an extended digital staring contest, recording each other’s mutual suspicion for posterity.)

In one sense, “My Scientology Movie” is as modest as its title implies: There are no grand revelations here, only interpretations of what we suppose, which are as amusing as they are troubling. Perhaps it’s the Brit’s deadpan sense of mischief that most vexes his targets. Humanizing a very private organization’s public face is one challenge; proving it no laughing matter is quite another.