It is exceedingly appropriate that "Mama" begins with the age-old story opener, "Once upon a time...," for the film wastes not a moment taking on the appearance and feel of a beautifully macabre fairy tale. Set in the real world yet fraught with foreboding whimsy, it's a supernatural thriller in the classic vein, relying on crafty suspense and an eloquently unfolding plot that aims to disconcert and frighten, then unexpectedly touch, the viewer. Executive-produced by Guillermo Del Toro—his stamp of approval is always reassuring—this curious tale of a specter's dangerous motherly love has been directed and co-written by Andy Muschietti, clearly taking great responsibility in expanding his much-acclaimed 2008 short to feature length. That he has a healthy budget, a top-notch technical crew, and the recently Oscar-nominated Jessica Chastain (2012's "Zero Dark Thirty") in the lead role only help his cause. "Mama" is an early-in-the-year scream, and deliciously so.

For five years, struggling artist Lucas (Nikolaj Coster-Waldau) has searched for his two young nieces, spirited away by his unhinged brother following the murders of two business associates and his estranged wife. Though hope of finding them alive has dwindled, he gets the surprise of a lifetime when 8-year-old Victoria (Megan Charpentier) and 6-year-old Lilly (Isabelle Nélisse) are found alone in a wooded cabin, malnourished and emotionally stunted, but nevertheless alive. Working with child psychologist Dr. Dreyfuss (Daniel Kash), the girls gradually improve their communicative and verbal skills until the day when the courts allow Lucas and girlfriend Annabel (Jessica Chastain) to take them home. With the opportunity to move out of a cramped apartment and set up in a suburban two-story under the condition they retain a certain anonymity, Lucas is gung-ho about taking care of Victoria and Lilly, while Annabel, a member of a rock band going nowhere, is more reluctant to take on the role of a parent. When Lucas is hospitalized after a bad fall down the stairs, however, Annabel is suddenly thrust into the position of the girls' temporary sole caretaker. The responsibility is far more than she bargained for, but one she is willing to fight for when the mysterious presence Victoria and Lily call "Mama" turns out not to be imaginary after all.

Intentionally or not, "Mama" takes worthwhile inspiration from 2002's "The Ring," 2003's "Darkness Falls," and 2011's "Don't Be Afraid of the Dark" while cooking up its own gothic standalone fable. The screenplay, credited to Andy Muschietti, Barbara Muschietti and Neil Cross, doesn't play coy when it comes to Mama's existence; she is established as very much real and something of a supernatural entity before the opening credits. Instead, the question becomes who, exactly, Mama is, where she came from, and why she wants Victoria and Lilly as her own. Annabel would be the first person to admit she's not matriarchal material—in her first scene, she very nearly jumps for joy when her pregnancy test comes back negative—but as she spends more time with Lucas' nieces and sees how troubled and vulnerable they are, her natural protective instincts begin to come out. Getting close to the girls, though, could also spell doom. "She'll get jealous," Victoria warns Annabel, and at the time Annabel only asks enough questions to know she'd better take a step back. When so many films of this kind demand their characters remain disbelieving cynics even as danger corners them from all directions, Annabel—and, for that matter, a sleuthing Dr. Dreyfuss—are quick to trust the girls and open their minds to the possibility that an otherworldly force is at work.

Jessica Chastain refuses to play weak characters or those that are informed by the men in their life, and so it goes with Annabel. Sure, she's the "girlfriend" of Lucas, but she's also very much her own person with her own ideas and feelings. With Lucas laid up in the hospital for the better part of the picture—arguably he's in there too long, one of the movie's more obvious plot devices—it is Annabel who is left seemingly alone with two children to take care of. Director Andy Muschietti, accompanied by Antonia Riestra's arresting cinematography and Anastasia Masaro's brooding production design, knows how to fill a screen with suggestive threats and thematically loaded imagery so that even an exterior establishing shot of an otherwise standard house is somehow transformed into a place few people would want to ever step foot in. At times, there are one too many set-ups ending in a jolt or pop-up scare, but these more predictable moments are offset by the control of Muschietti's clear genre knowledge and the restraint of a filmmaker who doesn't dare reveal all the cards he's playing with at once. Consequently, the anticipation of what's to come is one of its most effective assets, the sordid story of Mama and the crazed figure she came to be leading toward a third act that combines a rustic, cliffside mise en scene so imaginative and hair-raising it could make Tim Burton jealous with a sympathetic side that finds the tragic humanity within a monster.

Audiences who flock to see "Mama" will think they know where things are headed. On the way to this destination, there are obligatory scenes of characters slowly walking down darkened hallways, reaching to open doors they shouldn't, and sticking their noses in situations that could get them killed. What goes in between these conventions is where the film quakes, rattles and provokes, spacing out Mama's gasp-inducing reveals while slowing down to weave a layered narrative where one mother's mental instability and another's trepidations and inexperience ultimately lead them to a single common bond: their love of a child. Jessica Chastain is fantastic as Annabel, her hair shaggy and dark, her persona totally unlike any other character she's played, while Megan Charpentier (2011's "Red Riding Hood") and newcomer Isabelle Nélisse are equally transformative as children torn between domestication and the feral natures they've grown used to. And then there's the lady of the title, played to the same chilling hilt that spindly male actor Javier Botet put to unforgettable use as Nina Medeiros in 2007's "[REC]" and 2010's "[REC] 2." That Mama is a live-action creation, with CGI used only to complement in subtle, unobtrusive ways, makes her all the more perilously plausible. She's the stuff of nightmares, and what fairy tale worth its salt doesn't have a few bad dreams to go along with the hope of a happily ever after?