Chilean director Sebastián Lelio’s adaptation of Emma Donoghue’s 2016 novel is a completely absorbing descent into the mist – of 1860s Ireland, of mysticism, of religious fervour. Put aside the distracting framing device, two present-tense bookends assumedly meant to underscore our ability to suspend disbelief that end up undercutting this eerie, remarkable period film.

The vibe is off, the mood unsettling, from the moment we meet Florence Pugh’s Lib, an English nurse sent to a remote Irish village to observe (or, as many townspeople hope, to confirm) a reported miracle: 11-year-old Anna (an impressive Kíla Lord Cassidy) who has refused to eat for four months and yet remains curiously healthy. Lelio’s film approaches Anna’s anorexia mirabilis as a slow-burn mystery, as Lib grows fond of the girl and ever more frustrated with the inaction of those around her, people willing to accept sacrifice to preserve their preferred version of the story. It’s a thriller of fact versus faith that doesn’t judge the latter.

It also reaffirms faith in Pugh as a performer, in a thorny period role that recalls her breakout in Lady Macbeth. Pugh has yet to turn in a less than solid performance, particularly as a woman bristling at expectations, and she is utterly convincing here. Her Lib is fiery and steadfast, rational and susceptible as anyone to flights of desperate imagination. Pugh is almost terrifyingly self-possessed as a performer; as Lib’s frustration with her assignment and desperation to save Anna escalate, I found myself clenching my teeth, transfixed.

What could easily have become melodrama or a cheap trauma plot instead feels, in Lelio’s hands, strange and beautiful, shot through with the natural ruggedness of rural Ireland. The Wonder is an unusual journey, Lelio’s sharp direction never allowing what could be foreseeable revelations to tip into predictability. But it’s Pugh who’s the wonder here. Even with the framing device – yes, this is a story – you can’t help but believe her.