In ‘Nancy,’ a woman has a fraught relationship to the truth

★★55 Unrated. 87 minutes.

Andrea Riseborough might be the craftiest chameleon working in film. She’s the actress most likely to pique your attention as you desperately try to figure out where you’ve seen her before, whether she’s popping up as an insecure actress in “Birdman” or as Billie Jean King’s easygoing lesbian lover in last year’s “Battle of the Sexes.”

Neither of those personas show up — at least not entirely — in “Nancy.” Living in New Jersey with her ill mother (Ann Dowd), the character of Nancy Freeman is a blank slate, her energies given over to caretaking and traveling from one temp job to another. For entertainment, she adopts different online identities, developing relationships that are destined to keep her trapped in a drab, claustrophobic existence.

That all changes when Nancy hears about a couple in upstate New York who lost their 5-year-old daughter 30 years ago. When the local news broadcasts an age-progression rendering of what the child would look like today, Nancy becomes convinced that she’s that missing person. Her would-be parents are played by the wonderful J. Smith-Cameron and Steve Buscemi — wisely cast, since they each bear a credible resemblance to Riseborough herself.

In what looks like a badly styled black wig, her huge eyes seemingly permanently dilated, Riseborough personifies something both everyday and otherworldly in “Nancy,” which marks the feature debut of writer-director Christina Choe. Although the filmmaker structures the story as a modest psychological thriller, a brief reference to horror writer Shirley Jackson suggests creepier things to come. Choe keeps the audience unsure of whether we’re seeing a redemptive drama of self-discovery or a troubling portrait of severe decompensation.

The mopey, midwinter atmosphere of “Nancy” becomes increasingly and oppressively bleak, leavened only by Smith-Cameron’s spot-on portrayal of her character’s trembling, painfully fragile optimism.

As the depiction of a ghost haunting her own life, “Nancy” possesses an alert, tense sense of atmosphere, but it winds up being as glum and inert as the protagonist herself.