“Somewhere in the valley,” Peter (Christopher Denham) tells his girlfriend Lorna (Nicole Vicius), “there’s a woman living in a basement who says she’s from the future — and she’s amassing followers.”

Peter, an aspiring filmmaker, has enlisted Lorna’s help in making a covert documentary about the growing cult. They have passed themselves off as members, and Peter has swallowed a tiny device that records the images he captures with the tiny camera hidden in his eyeglasses. Everything he sees, he films.

But the more time they spend in the presence of Maggie (Brit Marling) — a blond woman of frail health who breathes through an oxygen tank and seems to be preparing her followers for some sort of mission — the more the couple falls under her sway.

“Sound of My Voice” is the astoundingly assured debut of director Zal Batmanglij, who also co-wrote the script with Marling and shows a remarkable talent for resourceful, low-budget filmmaking. The movie immediately draws you in with its opening sequence, in which Peter and Lorna are put through a specific ritual — handcuffed, blindfolded, transported by minivan, required to perform a ridiculously elaborate secret handshake — before they allowed to meet Maggie.

Played by Marling with just the right mix of charisma and vaguely sinister menace, Maggie is a hugely fascinating character. At first, you’re understandably skeptical of her claims. Her story about being from the future makes no sense, and she has no physical proof to back her claim. But the movie gradually seduces you into thinking, “Well, maybe …”

During one of the group’s meetings, a follower asks Maggie to sing a song — any song — that was popular in 2054, and she reluctantly agrees and sings The Cranberries’ “Dreams.” Aha! Busted, right? But when she’s told the song was actually a hit in 1993, Maggie counters by saying she didn’t know that, and who among the group could sing a song that had been popular 60 years ago?

“Sound of My Voice” has several stretches of tremendous suspense — in one scene, Maggie draws a painful childhood memory out of Peter that shocks his girlfriend and makes her wonder if he hasn’t subconsciously bought into the cult — and there are a couple of intriguing subplots, such as a little girl who spends her time at home playing with black Lego blocks, that imply the movie is building toward a revelatory resolution.

The filmmakers get you so invested in the build-up, the cop-out ending feels particularly disappointing. “Martha Marcy May Marlene,” another drama about a woman involved with a cult, ended abruptly, leaving you forever stranded in the protagonist’s state of emotional terror. But the way in which “Sound of My Voice” wraps up offers no catharsis — emotional or intellectual. When the movie’s over, you are free to interpret it however you choose. Was Maggie lying? Was she telling the truth? More importantly, why should anyone spend their time pondering this nonsense?

There is a huge difference between an unexpectedly sudden ending (like “The Sopranos”) and a non-ending. “Sound of My Voice” gets you all worked up and then leaves you hanging, as if the filmmakers couldn’t think of a way to solve the riddles they had created, so they opted for D) All of the above. Wrong answer.