Claire Foy, best-known for playing Queen Elizabeth on The Crown, substitutes royalty for insanity (or is she insane..?) in her latest feature, Unsane. Foy’s character, Sawyer Valentini, is a data analyst working in a bank who is left paranoid and emotionally shattered after a stalker devastates her life. Following a visit to a psychiatrist to chat about what happened to her, Sawyer becomes involuntarily committed to a mental institution. Her worst nightmare comes true when she discovers that the stalker (Joshua Leonard) is working there. With no allies and no one to believe her story, Sawyer is desperate to find a way out.

The career of director Steven Soderbergh, whose renowned works include the Oceans trilogy, Erin Brokovich, and Solaris, has been a fascinating one to witness. He hasn’t had a lot of luck of late though. His 2014 TV series The Knick got the axe after just one year while last year’s Logan Lucky, though a critical success, barely made a dent in the box office, garnering $47.4 million off a $29 million budget. Unsane sees the director expand his filmmaking ambition, not only because he shot and edited as well as directing the film himself, but also because he shot it on an iPhone 7 Plus in 4K.

The cinematographic effect is immediately striking and reminiscent of a project a film student might do, though of course in the hands of Steven Soderbergh, there’s not a hint of a gaffe in sight. Every moment of every shot is purposeful with the camera’s stasis complimenting the movie’s theme of entrapment. Regarding influence, Unsane takes inspiration not so much from One Flew Over the Cuckoo’s Nest or prison dramas as it does from Stephen King’s Misery.

Acting-wise, Claire Foy is exquisite in the leading role and there’s no reason to doubt she won’t have a brilliant career following her time on The Crown. Joshua Leonard, who’s probably best-known for 90s hit The Blair Witch Project, is an interesting casting choice for the stalker role and definitely pulls off the creepiness of his character (plus the beard helps). The rest of the cast aren’t major household names, aside from maybe Juno Temple, which works for the story. There is, however, one very distracting cameo from an old actor friend of Soderbergh which really takes you out of the movie.

Not without its plot holes, the dialogue alternates between natural and theatrical (writers James Greer and Johnathon Bernstein’s unimpressive credits include Jackie Chan flick The Spy Next Door, Lindsay Lohan’s Just My Luck and Larry the Cable Guy: Health Inspector). Unsane nonetheless moves at a sprightly pace and becomes increasingly terrifying as it progresses. It makes for a good thriller, and even when it does verge into B-movie territory, it does so in a very self-aware way. Whether the abstract format or Foy’s name being attached to it will prove attracting enough to audiences though is hard to call. For all its seeming strangeness between the unusual format, casting and title, it’s surprisingly conventional.