Director Steven Soderbergh (Side Effects, Erin Brockovich, Sex, Lies, and Videotape) seeks to question our perception of reality in this rather uneven psychological thriller, shot on an iPhone 7 Plus. Written by Jonathan Bernstein and James Greer, whose previous collaborations have been comedies, there are certainly no laughs here.

Unsane’s greatest asset is that it stars Claire Foy. She gives an outstanding performance, in a role that could not be further from the Queen, as Sawyer Valentini, a highly-strung, tough account manager who is unwittingly and involuntarily committed to a mental institution.

Sawyer has relocated to Pennsylvania from Boston and seems to be in control of her new life - that is until she has a panic attack during a Tinder date. Gradually, it transpires that she moved cities in order to get away from a man who had been stalking her for two years. In a therapy session, Sawyer reveals that she never feels safe and admits to having infrequent suicidal thoughts. Before she knows it, an arbitrary signature on a form has confined her into a secure facility, which is where her Kafkaesque nightmare begins.

The more angry, desperate and agitated Sawyer gets about her ‘wronged’ incarceration, the more her sanity is questioned and her stay extended. Initially, it’s not difficult to feel sympathetic towards her plight - even the police won’t help her.

She makes friends with kindly, fellow patient, Nate (Jay Pharoah), who manages to defy the rules by hiding his phone from the staff and allows her to contact her mother (Amy Irving), whose rescue effort is somewhat thwarted. Nate offers a plausible financial reason for how and why the facility manages to keep people prisoner inside it without any redress. Unlike Sawyer’s foe, crazy inmate Violet (Juno Temple), Nate appears to be the voice of rationality.

So far, so sane. Or is it? Once Sawyer says that one of the nurses, the suitably creepy George Shaw (Joshua Leonard) is actually her stalker things get murky. Is she mad, is her fractured mind experiencing a drug-induced paranoia or has being trapped in a sterile facility with mechanical staff made her go crazy?

Unfortunately, the film becomes totally unhinged in its final act and descends into lurid absurdity. For a psychodrama, Unsane is not unsettling enough, it lacks subtlety and its dramatic tension is too fragmented. Even Foy’s compelling performance cannot compensate for that.

Unsane will be released in cinemas from March 23