Is the young, beautiful blond woman truly a time traveler from a war-torn future, promising safety and enlightenment for a chosen few? Or is she merely a con artist who knows how to use her looks and magnetism to manipulate people for her own gain?

This is the question at the heart of "Sound of My Voice," one that you’ll be asking yourself until the very end and even afterward.

Brit Marling follows up on the promise of last summer’s "Another Earth," another sci-fi thriller that makes the most of its meager budget with intimate settings, well-drawn characters and steadily mounting mystery.

Marling co-wrote, co-produced and stars in both, and once again she leaves a strikingly naturalistic impression.

"Sound of My Voice," which Marling wrote with first-time director and fellow Georgetown University graduate Zal Batmanglij, is the stronger film of the two, though, with its gripping tension and relatable realism.

"Another Earth" benefited from an intriguingly fantastic premise, but you could easily imagine what goes down in "Sound of My Voice" — you could see how people allow themselves to get sucked into this sort of murky world.

Before we get to Marling’s character, though, we meet mousy Peter (Christopher Denham) and reformed party girl Lorna (Nicole Vicius), dating documentary filmmakers who have infiltrated a San Fernando Valley cult in hopes of exposing its leader, Maggie, as a fraud. The sequence in which they’re blindfolded, bound and stuffed into a van, then forced to strip, shower and dress in simple white clothes upon arrival is one that’s repeated with each visit, with increasing speed, in a beautifully fluid bit of editing.

Once inside the nondescript tract house, Peter and Lorna descend to the basement where they must prove they belong with an elaborate secret handshake; it may look silly at first but it matters later.

Then they meet Maggie in all her glory: a figure who’s simultaneously ethereal and imposing. Supposedly she comes from the year 2054 and is allergic to much of what constitutes present-day life, so she must stay downstairs, eat organic foods grown just for her and receive regular blood transfusions.

She’s got a mesmerizing strength about her, though, and it doesn’t take long for her to burrow into Peter’s brain and root out his innermost secrets in a quietly intense scene that’ll make you hold your breath. Marling is riveting in these moments, in the way she can shift emotional gears subtly and convincingly.

Lorna, however, remains skeptical. But once Maggie asks Peter to bring her a specific person from the outside world, it’s clear she’s not only powerful but also dangerous. The film never reveals her truth, though; you could argue your interpretation of her actions in a number of ways.

"Sound of My Voice" may seem like an extended episode of "The Twilight Zone," but within its clever structure it digs into the notions of identity, loyalty and the need to belong. It also raises the question of why there are no female cult leaders — besides Lady Gaga, of course.

But the way it does a lot with a little is the most hypnotic trick of all.