As anyone who has weathered a friend’s sudden, wholehearted embrace of veganism, Peloton, or QAnon can attest, fresh converts often possess an ecstatic fervor that many true believers spend the rest of their lives chasing. No one is as zealous – or insufferable – as a penitent sinner who has recently found God. Such is the case with Maud (Morfydd Clark), a private palliative-care nurse in the seaside town of Scarborough, North Yorkshire. Passersby could be forgiven for sparing Maud little more than a glance: the neat, tea-green uniform worn over a dowdy turtleneck; the auburn hair pulled back in a severe, plain-Jane ponytail; the perpetual, slightly disapproving frown. However, Maud is special. She has a secret, and not just the nail-studded portraits of Mary Magdalene she slips into the inner soles of her sensible canvas shoes. She is in direct communion with the divine, being subject to periodic euphoric fits during which God’s spirit seems to enter and permeate her quaking flesh.

This poses something of a dilemma for Maud: Should this holy boon be hidden away like a sacred treasure intended only for her, or should it be shared with the world as a testament to God’s boundless grace? As a recent convert, Maud is alert for the signs that might point her to the correct path. She seems to find one in the form of Amanda (Jennifer Ehle), a celebrated modern dancer and choreographer who is dying of late-stage lymphoma. As Maud’s latest charge, Amanda is a prickly libertine who rather unexpectedly warms to her new nurse’s sturdy, pious manner – at least at first. Maud, for her part, is eager to show her wealthy, bohemian patient a more spiritual way of living (and dying), but this would require Amanda to abandon her decadent inclinations in favor of self-denial and mortification. Maud’s brushes with the divine might resemble orgasmic seizures, but she’s seeking a higher form of bliss, one glimpsed between the starbursts of pain as she kneels to pray on scattered popcorn kernels.

Saint Maud, the impressive and thoroughly unnerving debut feature from British writer-director Rose Glass, is attuned to the agony and ecstasy of faith to a degree not seen since Paul Schrader’s First Reformed (2017). Unlike the Rev. Toller, however, whose haunted, strangled interiority was crucial to that film’s power, Maud is something of a cypher. The spiritual and physical anguish she endures is palpable, but Glass refrains from revealing much about the young woman’s troubled past or her peculiar, personal theology. Maud speaks in voiceover, but there is a purposefully cagy quality to her auto-confessional monologues, as if she were penning a diary that she assumes will one day be perused by strangers. Brief flashbacks point to a bloody inciting incident at the hospital where Maud once worked – an event that seems to have triggered her abrupt conversion from a bad-decisions party girl to an ardent follower of Christ. Glass’ approach to her protagonist favors dread-choked mystery over a clear elucidation of Maud’s motives and worldview. Which suits Saint Maud just fine: This is, after all, a horror film, one in which a gnawing terror undergirds the potent yet vague sensation that Maud is destined for great things. Terrible things, but also great.

Saint Maud runs a lean 84 minutes, but Glass’ rather ingenious screenplay comfortably fits a three-act structure into that running time through a dogged focus on her protagonist’s dark psychological journey. The emergence, maturation, and dissolution of the bond between Maud and Amanda all unfold in the first 30 minutes or so, setting Maud up for a feverish spiritual crisis and an eventual reaffirmation of her faith. She might be a zealot, but as the late John Le Carré's spymaster George Smiley once observed, “The fanatic is always concealing a secret doubt.” Maud is plagued by visions of a maelstrom, whether in sink drains, glasses of ale, or the ashen Yorkshire sky. Although the origin of this omen in Maud’s private mythology might be obscure, its meaning is clear: chaos, darkness, and oblivion. Of course, keeping the Devil at bay often requires penance. By the time Maud hears a cockroach speak with God’s voice and then starts filling plastic containers with holy water and nail-polish remover, it’s plain that He will demand more than a few Hail Marys.

Glass has plainly absorbed some of her aesthetic from the past decade’s worth of art-horror landmarks, drenching her debut feature in the sort of severe, skin-crawling ambience that has practically become synonymous with distributor A24. Cinematographer Ben Fodesman makes fantastic use of the film’s Northern seaside locations, conveying an atmosphere that blends Wuthering Heights gloom with the late-capitalist despair and dinginess of Ken Loach’s features. The film’s setting is crucial: It allows Saint Maud to juggle both the gothic and realist without a trace of dissonance. High on the hill, Amanda the worldly artiste lies dying in her musty manor, surrounded by faded glories – but from her bedroom window one can make out the town’s seedy pubs, arcades, and takeaway stands on the waterfront below. It works surprisingly well, mostly because the filmmakers treat both the creaking staircases and the crumpled chip wrappers with the same ominous weight.

Saint Maud thrives on more than atmosphere, however. Glass approaches every shot with an uncommon mindfulness for how mise-en-scène can be employed to create layers of meaning. Maud’s dismal one-room flat, for example – repeatedly established with a wide shot that emphasizes its barren, coffin-like finality – at once suggests financial hardship, monastic asceticism, and off-putting eccentricity. This indefinite, multidimensional quality is echoed in Clark’s remarkable performance, which manages to feel both uncomfortably raw and utterly inscrutable. It’s always plain what Maud is feeling, but the why – and how she might react to any given situation – is ominously uncertain. When an old friend drops in on Maud’s apartment, there’s no obvious threat, but the scene nonetheless tingles with queasy tension. Like a newly anointed New Testament shaman, Maud is teetering on the precipice between the visible and invisible. She can see the evil of the fallen world thrumming all around her. There is no predicting what might provoke her, or what she might do to safeguard her soul.

Like many horror films about the pathological extremes of religious faith, Saint Maud presents its protagonist as potentially unreliable, permitting an insidious undercurrent of uncertainty to snake through the story. Is this a film about mentally ill woman who desperately needs intervention before she harms herself or someone else? Or is Maud truly experiencing communion with some divine power? Given how closely the story follows Maud’s viewpoint, even the film’s most florid moments of post-Exorcist religious horror could theoretically be chalked up to hallucination. The rigor of Glass’ writing and direction cunningly moots questions regarding what is and is not real. Saint Maud isn’t a horror film about madness or God, but about suffering, about the vast, twilit country between blazing certainty and devouring doubt. Maud might be a mystery, but her anguish is as real as blood and flame.

Rating: B+

Saint Maud will be available to stream from Epix on Feb. 12, 2021.