Since 1963, the Austrian birthrate has halved. You can't blame Severin Fiala and Veronika Franz's new thriller, Goodnight Mommy, for the trend, but it sure isn't helping.

The quiet creepshow follows eleven-year-old twins Lukas and Elias (Lukas and Elias Schwarz, great), who suspect their mom (Susanne Wuest) wishes they hadn't been born. We can see her point. Though the film avoids exposition, from the few crumbs we can snatch, Mother (the only name we get) is a divorced TV game show hostess panicking about her looks. (Game show hostesses know it's not their talents that keep them employed — just look at Vanna White's astounding youth.) She's shuttled the boys from Vienna to the countryside so she can recover in privacy from plastic surgery. It's a vain woman's hideout, all gray walls, dim light, pretty dolls, and aspirational mannequins. Mother instructs the kids to tell visitors she's ill and then retreats into her dark bedroom with her bruises obscured in gauze.

The twins don't care about TV ratings. They don't care about staying quiet so Mother can sleep, rocking themselves silly with burping contests in the swinging bubble chairs downstairs. And they barely care about their mother's feelings. At night playing a guessing game, the boys write "Momma" on a Post-it stuck to her bandaged forehead. "Am I famous?" she asks. "Well...sort of?" they reply.

Mother never knows that she's "Momma." Pretty soon, Lukas and Elias aren't sure she is, either. Would their real mom refuse to speak to Lukas? To give him dinner? Lukas is certain this mummied monster is a fake, and Elias is unable to say otherwise. When they ask her to name Lukas's favorite song, she's so self-centered that she can't. Even if she unwrapped herself, what new face would they see? Since we've never met her — and Goodnight Mommy smartly doesn't do flashbacks — we can't be sure. Yet from Wuest's haughty carriage, we are sure that we don't like Mother one bit.

Fiala and Franz see the humor in setting their sibling spook story in rural Austria, home of the movie-famous von Trapps. Goodnight Mommy opens with a red-cheeked blond family belting an ominous lullaby: "Tomorrow morning if God deems, you will wake from your dreams." The von Trapps dreamed of their father marrying the saintly nun Maria. This film plays as if Lukas and Elias got stuck with that spoiled, beautiful countess. At night they huddle in their bunk beds and listen to a cassette of their real mom singing them to sleep. Meanwhile, the woman in their house strips naked in the forest and screams.

Much of this doesn't make sense. Goodnight Mommy operates on kid logic: random obsessions, athletic digressions, subplots that wander away. Halfway through the movie, the boys stumble upon a cave packed with human skulls. You might expect a twist — but they never go back. Instead, Fiala and Franz make us feel their boredom. Trapped inside, these slender, wide-eyed redheads climb over each other like newts in a tank. But outside is a playground: thick cornfields, mud as springy as trampolines, and even an actual trampoline, on which the boys bounce so exuberantly during a thunderstorm that, like a mom, I was panicked one of the actors might break a neck. No wonder they drag their feet coming home.

But the film is more concerned with tone than tension. This is all building to a showdown — a violent, retching, kid-torture final act — but we're only invited to be distant observers. As the boys and their mother divide the house into battlegrounds, things we mistake for clues prove just to be decorative gags. Nothing adds up, although it isn't clear how much we've been misdirected until the lights come up and the audience, at least in my theater, shuffles out in a daze, cracking jokes about having to go home to their children. Much of the confusion could be defended as the product of the leads' immature viewpoint — even the most logical kid is only half-certain that boogeymen don't live under the bed, so perhaps it's OK that the movie, too, hedges its bets.

Goodnight Mommy is a well-crafted cheat with a killer punch. The ending is a knockout, though it absolves you from remaining invested in everything before, and the ideas that interest Fiala and Franz are primitive and scary. What are the limits of love? What trust does blood deserve? And if three people tied together by genetics can become strangers, what bonds aren't breakable?

Goodnight Mommy
Written and directed by Severin Fiala and Veronika Franz. Staring Susanne Wuest, Elias Schwarz, and Lukas Schwarz. Rated R. 99 minutes. Opens Friday, October 9 at O Cinema Wynwood, 90 NW 29th St., Miami, o-cinema.org.