PARK CITY, Utah — A troubled young woman (Andrea Riseborough) decides she’s a long-lost kidnapping victim in the drama “Nancy,” which just won the Waldo Salt screenwriting award for writer/debut feature director Christina Choe at Sundance. It’s an impressively disturbing performance from Riseborough, if ultimately more character study than fully formed plot.

From the moment we meet her, it’s hard to get a grip on what the deal is, exactly, with Nancy (Riseborough), who cares for her ailing mother (Ann Dowd) while maintaining what seems like at least a couple of internet alter egos.

She meets a man (John Leguizamo) for an online date wearing a fake pregnancy belly; she tells the staffers at her dentist’s-office temp job that she’s just back from a fun vacation in North Korea, augmented with dubious photo evidence on her phone (“This is . . . just a field”).

She’s visibly in need of mental health treatment, and possibly suffering from a trauma of some sort. She’s vulnerable but also a practiced liar. The difference between her truths and her fictions is known only to her — if even.

When she catches an age-progression photo of a girl kidnapped decades ago that looks strangely familiar, Nancy believes — or does she? — that it’s an answer to why she’s always felt like such an outsider. Riseborough, with her mane of unbrushed hair and sunken-eyed pallor, puts her frown-etched face up against the sunnier Photoshop picture and sees a version of herself she’d like to think is the more real one.

Once she shows up at the home of the kidnapped girl’s parents, played by J. Smith-Cameron and Steve Buscemi, they alternate between suspicion (especially Buscemi, for whom side-eye comes as easy as breathing) and faint, painful hope.

Riseborough communicates such intense internal confusion that she can will herself into facial tics. You watch her, rapt, waiting for the other shoe to drop — and even if that drop is less than satisfying, you’ll come away knowing you’ve seen a master at work.