Within the context of 1956's German family feature "The Trapp Family" (a key inspiration for 1959 Broadway musical "The Sound of Music" and its 1965 film adaptation starring Julie Andrews), the sight of Ruth Leuwerik singing Johannes Brahms's "Wiegenlied: Guten Abend, gute Nacht" surrounded by her character's adorable brood of stepchildren makes for a heartwarming cinematic adieu. When isolated and made the prologue of ominous dark fable "Goodnight Mommy," this innocuous clip is suddenly enshrouded by a distinctly unsettling foreboding as Leuwerik croons the line (translated from German), "Tomorrow morning, if it is God's will, you will wake once again." Long before writer-directors Severin Fiala and Veronika Franz get through with their audience, this gentle lullaby will more closely resemble a threat of shuddersome woe.

In an idyllic patch of remote Austrian countryside, 10-year-old twins Elias (Elias Shwarz) and Lukas (Lukas Schwarz) live with their mother (Susanne Wuest) in a hyper-modern, impeccably furnished dwelling on the edge of a large wood. The brothers busy their days playing cornfield hide-and-seek, exploring caves, and lazing around in a nearby lake. When they return home, they are more hesitant, even suspicious, of the person waiting for them. Her head wrapped in thick, white gauze, Elias and Lukas' mom has taken a suspected leave of absence from her news anchor job to recuperate from surgery. The brothers notice she isn't her old self—she asks for strict peace and quiet, for one, and is certainly more aloof toward Lukas, with whom she is unhappy for reasons unknown. During a game of Twenty Questions, she acts as if she is unaware of her career when the boys choose her as their mystery answer. Unleashing one of their pet cockroaches on her bed, the boys watch as she doesn't so much as flinch as the bug disappears inside her mouth. With no one else around—the closest town is all but entirely desolate, save for a priest (Hans Escher) and an accordion player (Erwin Schmalzbauer)—Elias and Lukas are determined to worm the truth out of the imposter they believe is living with them.

"Goodnight Mommy" is deliberately paced but uncommonly arresting, its leisurely tempo anything put symptomatic of narrative slackness. As tight and unrelenting as a pair of too-small handcuffs, the storytelling embraces the "is-she-or-isn't-she?" conundrum at the film's center while trading on the identity of its protagonist. The mother, lurking inside a home of modern conveniences, sharp corners, and out-of-focus, black-and-white portraitures, appears to be masking more than her face. Since the bulk of the picture is told from the boys' perspectives, the viewer is inclined to agree with them that something seems off about her. For their parts, Elias is frustrated at his mom for treating Lukas with what he deems as passive-aggressive negligence, the two of them turning to rebellion and sleuthing when they think she's not looking. Their mistrust leads to a progressively heightened power play, one that guides all three characters down some harrowing avenues of harsh self-realization in the second half.

In lesser hands, it is easy to imagine how this premise could be twisted into an obvious horror tale, exploitative in its treatment of young Elias and Lukas and reliant on figures popping out of the dark. Filmmakers Severin Fiala and Veronika Franz are operating on a considerably headier, more thematically loaded level, using the unease of their haunting mystery as backdrop to a heartrending illustration of psychological trauma, vanity and loss. The complexity of what they have achieved is startling the longer one thinks about it, each veraciously structured tableaux playing differently between first and subsequent viewings as the piercing reality of this mother and her children reveals itself. Lending further weight to the proceedings are the performances. Newcomers Elias and Lukas Schwarz are virtual phenoms, captivatingly honest in the face of emotionally raw and demanding material. Susanne Wuest is perfectly cast as either their convalescing mom or a nefarious intruder, the actress' ethereal aura, sinister edges and mummy-like bandages making for a wickedly chilling blend that leaves one consistently torn over her intentions.

The third-act revelations "Goodnight Mommy" imparts are not all novel in conception, but the profundity of its implications linger and provoke. Director of photography Martin Gschlacht (2009's "Revanche") finds a childlike wonder and menace in his every frame, taking full advantage of his pastoral exteriors and the antiseptic loneliness of the house's spatial architecture. Bleak yet ultimately empathetic, the enigmatic story ultimately works itself out without spelling things out, finishing with a punch to the gut that avoids gimmickry by genuinely having a point; its existential pathos both rejuvenates and shrewdly recalibrates all that has come before. By dropping breadcrumbs along the way, some of them leading to dead ends and others astutely speaking to the reverberations of tragedy and the fragile impressionability of children, "Goodnight Mommy" constructs an alarming fairy-tale diorama frozen in a state of peril and humanity.