It is something of a cliché, but the phrase is so apposite in the case of Unsane that I will permit a temporary suspension of my scruples. This film loses the plot. 

For an hour or so, it holds one rapt, not with the question ‘is she?’, to which the answer, it is fairly immediately clear, is a resounding ‘no’, but with the time-honoured questions of ‘why?’ and ‘how?’, as well as my personal favourite, ‘what would you do in the same situation?’ The scenario it presents its audience with is a horrifying one, each new revelation inflaming a profound sense of injustice, as well as one of panic, as the prospect of a happy ending seems to grow ever more distant.

We are given only a brief introduction to Unsane’s protagonist, yet Claire Foy manages, with aplomb, to craft a credible and broadly sympathetic character, which makes her fate a suitably cruel one, and allows us to marvel at the deviousness of her oppressors. 

Jay Pharoah is similarly magnificent as a recovering opioid addict who seems practically the only other reasonable human being in the picture. The story unspools neatly, at a sharp pace, and is unafraid to twist the knife it has planted in our backs. The urgent score ratchets up the tension, and the tight, pallid, slightly fish-eyed iPhone photography hems the audience in, and continues to unsettle. Then – well, the wheels start falling off.

The first murder is enough to tip the film over the very thin border which stands between an unnerving thriller and a preposterous one. Things stop falling neatly into place and start being forced in with unwanted helpings of blood. The shoddy sound recording gets shown up by a slagging match, and rather than inducing claustrophobia, kills the encounter’s intensity. The film morphs from a satisfying conspiracy movie into an unsatisfying psycho killer movie, in which the psycho killer is not all that menacing, nuanced, or entertaining.

The sensation of witnessing potential greatness dissolving into a colourless solution is always infuriating when compared to the sensation of witnessing sustained mediocrity, and it should be emphasised that the first two thirds of Unsane are very effective indeed. If two thirds of a film are going to exceed the remaining third, however, I would always rather that these were the last two thirds than the first.

As such, it should also be emphasised that Unsane feels overstretched even at a mere ninety-eight minutes, and is likely to leave its viewers, if not angry, then just disappointed. It loses the plot, all right, but more in a literal sense than a figurative one, and in a manner far more redolent of careless negligence than mad genius