Warning: This review refers to key plot points. Check out of a screening before reading.

Steven Soderbergh once promised to retire and for a time did, perhaps because the indie filmmaker in him felt constrained by the trappings of his success. He had access to professional equipment and crew, A-list actors and with it, the expectation that he should make flashy mainstream fast food like Oceans 11, when his heart burst for another Sex, Lies and Videotape.

Now, sucked off by developments in camera phone technology, he’s gone all edgy and shot an entire thriller on an iPhone. Don’t ask me how it lit the thing – it can’t surely all be natural light, and let’s be grateful that the homemade aesthetic is buoyed by creative mise-en-scene – giant glasses of beer in the foreground, monster close ups, low angles – in fact the full filmmaker’s songbook. But all you want to know is, given the ubiquity of the technology, and mindful of the obvious caveats – namely the availability of acting talent and a budget to book locations, is Unsane a movie you could have made or something better?

It’s better. In grabs – great even, but informed by a tension larger than that generated by Claire Foy’s pseudo-Kafktaesque predicament – internment in a mental health hospital, trapped with the man – her stalker, Joshua Leonard, that made her seek psychiatric help in the first place. It’s a circular idea, like the rotation of a negative thought; the kind of mind splinter that generates the fear, depression and anxiety Foy’s character wrestles with. But it’s a horror-thriller premise built around a plot hole, which once considered, acts like the after-image of Foy’s stalker. Wherever you turn, it’s there.

For the movie to work you have to accept that Leonard, having followed Foy to her new home in Pennsylvania, and with great prescience, anticipated the private mental hospital she’d refer herself to for psychiatric consultation. Then, with further foresight, he gambled they’d trick her into admitting herself in a bid to rip off her insurance company, so killed an orderly, months ahead of time, assumed his identity and applied for and got a job at said hospital, passing the background check, giving him both closed access to his quarry and the means to drug and intimidate her.

It could be a coincidence of course, maybe his father’s death at a hospice gave him the impetus to seek a job close to Foy, in a caring role, but this clockwork plot tells us that Soderbergh was so enamoured by the core set up that he didn’t really give a fuck whether it made sense or not. Once we know Foy isn’t hallucinating and is trapped with her foe, Unsane becomes a different type of movie; low-budget schlock with a grounded tone.

Said tone gives the film a sense of reality and grit, despite the compounding implausibilities, making it unnerving and uncanny throughout. Those on gender politics watch will lament the disproportionate suffering metered out to female characters, not all of it inevitable despite the stalking theme, while enjoying the righteous fury of Foy’s character and the manner in which she takes the fight both to the hospital and her tormentor. An unusual and memorable thriller then, well worth the $800 Soderbergh spent on his camera phone.