The one great blessing of Zal Batmanglij’s debut feature, about a couple of would-be documentary filmmakers who get initiated into the inner sanctum of a cult leader in order to film an expose on her scam, is that the writer-director resisted the temptation to go the dreaded “found footage” route. “Sound of My Voice” follows a conventional narrative model, and though Rachel Morrison’s camerawork has a homespun feel, it doesn’t embrace the ultra-jerky style so often employed today. Otherwise, however, the film is an overextended treatment of a pretty threadbare premise, told with a sense of self-importance the story doesn’t warrant.

The interlopers are Peter (Christopher Denham), an elementary school teacher with ambitions to do something important, and his significant other Lorna (Nicole Vicius), whose past addiction has led her to flit from one dream to another. We first meet them following elaborate directions about connecting with the cult’s messengers to be brought to the basement headquarters of Maggie (Brit Marling), a mysterious young woman who claims to be from 2054 and makes ambiguous remarks about a coming war to her adoring acolytes. The initial sequence showing the couple brought blindfolded to the location and being purified, robed, and proven worthy of admittance by an elaborate secret handshake is genuinely unsettling, and when Maggie arrives, attached to various medical paraphernalia, and begins quietly intoning alternately calming assurances and slightly menacing prophecies, the effect is considerable, largely because of Marling’s artful performance.

But before long the impact dissipates as Maggie’s bromides grow stale. A nifty moment, when a follower asks her to sing a song from the future and she responds with one that’s obviously an oldie even now, changes the mood briefly (and leads to the abrupt expulsion of a listener who points out the fact), but it isn’t enough to break the monotony for long.

And it’s soon followed by a curious plot turn when Maggie takes Peter aside to break down his reserve and, eventually, to lay a special task on him: he’s to bring one of his students, a young girl (Avery Pohl), to meet with her. Lorna tries to persuade him not to agree, quite logically arguing that it would amount to kidnapping. But Peter seriously considers going through with it in order to keep the project going. His decision is effectively short-circuited when he’s approached by a Justice Department official to help them capture Maggie, using the girl as bait. That leads to a final twist that won’t be revealed here but is meant to leave a mood of studied ambiguity.

Unfortunately, what this whole business does is to raise an uncomfortable question: is the fact that this particular girl was in Peter’s class a very unlikely coincidence? Or was Peter deliberately lured into the cult because he had access to her—something the initial scenes don’t support? Or is the girl just selected randomly in order to test his loyalty (a possibility the ending appears to rule out)? Presumably one’s supposed to overlook these questions, simply carried along by the overall plot. But the film’s lethargic pacing and stumbling gait derail that hope.

Nor does the acting help things. Apart from Marling, who brings a canny mix of the persuasive and the misleading to Maggie, the cast doesn’t impress, with both Denham and Vicius coming across flatly and the lesser roles only adequately handled.

The picture is clearly a low-budget affair, but it looks reasonably good; the problem with “Sound of My Voice” isn’t so much in technique as in script. One can imagine it as a fine half-hour “Twilight Zone” episode. But as a feature, even at a mere 84 minutes, the length makes the thin, shallow idea feel laboriously pretentious.