“I must be insane,” whimpers Sawyer Valentini (Claire Foy) in Steven Soderbergh’s psychological thriller Unsane.

It is a powerful, effectively delivered line that encapsulates the pervasive sense of fear and unease that runs through Soderbergh’s latest directorial outing.

The film focuses on Sawyer who, despite relocating to a new city, still suffers from the trauma inflicted upon her by stalker David Strine (Joshua Leonard).

Following a therapy session Sawyer finds herself involuntarily admitted to a mental institution, where she is plagued by visions of her former stalker who may have followed her there.

Soderbergh’s decision to shoot his film entirely on an iPhone 7 Plus is a masterstroke.

The technology lends itself to the claustrophobic confines of the mental institution in which Sawyer is trapped and accentuates the deep sense of disquiet and foreboding created by Thomas Newman’s eerie, pulsating score and James Greer and Jonathan Bernstein’s sharp, darkly humorous script.

The performances are universally strong too, with Foy impressing the most. Playing a character radically different from her quiet, dignified turns in The Crown (2016-present) and Wolf Hall (2015), the actress delivers a ferociously intense performance as the emotionally and physically battered Sawyer, effectively keeping the audience guessing whether David’s presence is real or a symptom of her trauma.

Leonard is also effective, playing the obsessive David not as a monstrous caricature but as a sad, delusional and pathetic individual.

Driven by Foy’s blistering central turn and Soderbergh’s strong direction Unsane is a compelling and unnerving thriller.