Is religious fanaticism a form of mental illness? Certainly people such as Richard Dawkins and Daniel Dennett would argue it is. However, from the perspective of the fanatic, such fanaticism is often not only logical and justified, but unavoidable; they don't choose to be fanatical, they are compelled to be fanatical. The disparity between what a fanatic believes and what other people believe is the main issue examined in Saint Maud, the stunning debut feature from writer/director Rose Glass. Part-horror, part-psychological thriller, part-character drama, part-ecclesiastical treatise, Saint Maud can be read in a variety of ways - an analysis of the interaction between faith and self; a threnody for the life of a young woman suffering a mental breakdown; a drama about loneliness; a tale of possession; a tragedy about the frailty of the human body. Told mainly (although not entirely) from the perspective of a fanatical Christian, the story makes room for the possibility that, however unlikely, such fanaticism isn't mental illness at all and that God really is communicating with this person. And this magnificently handled ambiguity is the film's trump card. Disturbing, horrifying, challenging, unpredictable, emotional, and occasionally very funny, this is a film that forges a path entirely its own, and is as impressive and daring a directorial debut as you're ever likely to find.

In a thoroughly depressing English seaside town, Maud (an incredibly physical performance from Morfydd Clark) is a recent convert to Roman Catholicism. Exceptionally devout, she believes that mankind is amoral, lustful, and wicked, and that only by way of a true saviour can we be saved. Is she that saviour? It's possible, because God has explicitly told her that He has very special plans for her in the near future. Meanwhile, Maud is working as a private palliative care nurse, and the story begins as she arrives for her first day with Amanda Köhl (the always brilliant Jennifer Ehle); a formerly world-famous American dancer and choreographer suffering from end-stage spinal lymphoma. She and Maud get on well - Maud admires her strength of character and zest for life, whilst she wants to help Maud let her hair down a little. However, there are certain elements of Amanda's life of which Maud does not approve; most significantly, the frequent visits from Carol (Lily Frazer), Amanda's lover. When catastrophe strikes and a dark secret from Maud's past threatens to resurface, Maud decides to prove to Amanda, God, and everyone else just how far mankind has fallen and just how sanctified she really is.

Although Maud is a hard-line fundamentalist, Glass refuses to dismiss her, arguing instead that such individuals genuinely believe they really are communicating with the Divine - Maud may be mentally ill, but even if that is the case (and the film is in no rush to confirm that it is), then surely she deserves compassion and kindness, so completely has her mind bent reality to support her delusion. Glass tells much of the story from Maud's subjective perspective, and in this sense, it's almost understandable when she sees signs of God's presence in everyday things (an inexplicable whirlpool in a glass of beer, for example) - this may be delusion, but if it is, it's a total delusion that she is powerless against. In a very real sense, she cannot be held accountable for her actions.

Even irrespective of mental health issues, however, Maud is all-in on the whole Catholic thing. She tells God, for example, about how important her work is, as it allows her to "save souls" and she credits her recent conversion to Catholicism as reversing the downward spiral of her life. She's also a firm adherent of the Job school of faith-by-suffering, cheerfully telling a beggar, "never waste your pain" and later engaging in some truly gnarly DIY shoemaking.

Along the same lines, she tolerates Amanda's little digs about her life and how lonely she seems, but when Amanda turns her caustic wit to Catholicism, Maud is unable to let that stand without offering rebuke. Her relationship with Amanda forms much of the film's narrative backbone, with neither woman allowed to occupy the moral high-ground. Amanda is profoundly bored with her illness, and her isolation and inability to leave the house mean she seizes on this strange, ultra-serious young woman who has come to look after her. Amanda is not a villain any more than Maud, but she does regard Maud as a plaything, not with the intention of hurting Maud, but with the intention of amusing herself.

As strong as Saint Maud is thematically, however, where it really excels is in its aesthetic design. Glass directs the hell out of it, and there's not a weak link amongst her crew - from Ben Fordesman's murky cinematography to Paulina Rzeszowska's detailed production design to Paul Davies's oppressive sound design to Adam Janota Bzowski's creepy score to Mark Towns's ambiguous editing (including a shocking slam cut right at the end that's as brilliantly jarring and thematically crucial as anything in the work of Nicolas Roeg).

Crucial to the overall aesthetic is how Glass handles perspective; most (although, crucially, not all) of the film is told from Maud's perspective, so we encounter her visions not as an objective third-party would, but as she does. So, when she sees a small whirlpool spontaneously appear in a glass of beer, we see the same thing, and there's no cutaway to show us Maud staring at a normal glass; when a towel placed near a crucifix falls to the ground for no obvious reason, we see it as she does, and there's nothing to objectively suggest why it may have fallen; when God talks to her (in Welsh, no less), we hear His voice as she does, and there's no portion of the scene where we see Maud answering a voice we cannot hear.

Along the same lines, what are we to make of the many (many) shots of Maud with windows or lights in the background that create a halo effect? Or of the shot of her walking on the beach, with a thin layer of water covering the sand, which is framed in such a way that it looks like she's walking on water? One particular scene near the end of the film, which I won't go into as it would be a spoiler, is especially important in the construction of a subjective point of view - what we're seeing couldn't possibly be anything other than psychosis, and yet the film has given us very little to confirm such a reading. Could it be that what Maud is experiencing is real? Is this scene confirmation that her mind has irreparably snapped, or is it confirmation that she was completely sane all along? Constructing a scene based on two literally inverse interpretations can't be easy, yet Glass does it so smoothly, you won't even realise the sharp dichotomy until it's all over.

Running only 84 minutes, it's extraordinary how much Glass squeezes into her debut feature; from the arresting performances by Clark and Ehle to the thematic complexity to the extraordinarily well-handled perspectival ambiguity to the haunting aesthetic design. Looking at issues such as trauma, faith, fundamentalism, sexuality, and human impermanence, the film has much more going on than the generic horror elements one might expect. Either a depiction of the mental collapse of a young woman or a study of the supernatural, the film is built on ambiguity. One of the best directorial debuts I've seen in a long time, I was only half-way through and I was already looking forward to whatever Glass does next. Saint Maud probably won't break any box-office records, but we are going to be hearing a lot from Rose Glass in the future.