Dan (Chris Beetem) and Joyce Thompson (Susan Pourfar) are heading out to a nice dinner to celebrate their 13th wedding anniversary. When their regular teenage babysitter Maggie (Elizabeth Jayne) is unavailable, she refers them to her friend, Anna (Sarah Bolger). Dan and Joyce wouldn't normally bring a stranger into the house to watch their kids—11-year-old Jacob (Joshua Rush), 9-year-old Sally (Carly Adams), and 4-year-old Christopher (Thomas Bair)—but they trust Maggie's judgment and Anna seems perfectly pleasant once they meet her. What they (and Maggie) do not know is that the Anna who arrives at the Thompson residence is actually Emelie, having stolen the real Anna's (Randi Langdon) identity following her abduction and possible murder.

A deliberate, nerve-jangling exercise in uneasy tension, "Emelie" takes the confined space of a single house and milks its frightening premise for all it's worth. Rich Herbeck's tautly designed screenplay and Michael Thelin's perceptive directorial prowess turn the "When a Stranger Calls"-esque babysitter-in-peril concept on its head, pitting a trio of increasingly suspicious youngsters against a person in charge who isn't at all who she claims to be. Who Emelie is and what she wants shall be left for the viewer to discover, but from the opening scene Thelin makes clear she has taken on a new persona, boldly waiting to be picked up by Dan outside her latest victim's home. Once alone with Jacob, Sally and Christopher, Emelie reveals worrisome child-care methods. Initially working her way into their good graces by allowing them to do things usually off-limits, she edges ever closer to crossing the line as more sinister intentions take over.

"Emelie" tells of a supremely unorthodox rite of passage as pre-teen Jacob must find the strength and maturity inside himself to protect his younger siblings from an encroaching threat lurking among them. His battle of wills against Emelie forms the arresting muscle of this deceptively minimalist story. What unfolds is psychologically riveting and never two-dimensional, with Joshua Rush (2012's "Parental Guidance") a terrific, unaffected find as Jacob and Sarah Bolger (2015's "The Lazarus Effect") essaying a title villain who is anything but one-note. Wounded by a tragic past, the desperate, unhinged Emelie proves exceedingly plausible, which makes her—and her ultimate motives—all the more haunting. Bolger's multilayered reading of this fascinating role is equaled only by how ideally cast she is, able to appear sweetly innocent one moment and legitimately menacing the next. As Sally and Christopher, newcomers Carly Adams and Thomas Bair are naturals in their own right, handling emotionally difficult material and finding truth in their reactions.

Part character study, part cat-and-mouse potboiler, "Emelie" ratchets suspense through thematically loaded suggestion and plaguing disquiet from its disturbing ambiguity. If there are occasional ignored exit signs—that is, obvious means of escape and modes of sending for help that would significantly shorten the already crisp 82-minute running time—the film at least realistically acknowledges its child characters and their frame of mind as they try to figure out if there is immediate danger in their situation. The third-act showdown is, perhaps, overly convenient, but also involving enough to place one firmly on the edge of his or her seat. "Emelie" may seem straightforward in its plotting, but there is nothing cut-and-dry about director Michael Thelin's enticingly crafted narrative and Sarah Bolger's uniquely chilling antagonistic figure. From start to finish, "Emelie" is a smart, crafty little thriller.