Film critics took notice last year when Steven Soderbergh returned to directing films after a four-year hiatus he had initially announced as a retirement. Logan Lucky seemed tailor made for the director of Ocean's Eleven and its two sequels, even if the mystery screenwriter is not, as many suspect, actually Soderbergh's wife Jules Asner. But Soderbergh, whose love of cinema is both extensively self-documented on his personal website and evident in the appealing films he makes, is already back at it. Barely six months after the acclaimed but under-attended Logan Lucky, Soderbergh picks up another directing credit in the thriller Unsane.

Thematically, this psychological thriller recalls Soderbergh's last pre-hiatus theatrical release, 2013's Side Effects. In scope, though, it may be more akin to Soderbergh's under-the-radar projects like Bubble and The Girlfriend Experience. Unsane was shot on an iPhone 7 Plus and produced for a mere $1.2 million. Though it is opening in theaters wide, it is from the habitually stunted Bleecker Street (for whom the underperforming Logan Lucky stands as their by far biggest and highest grossing release) and lacks the star power of Channing Tatum and even Rooney Mara.

Claire Foy (apparently best known from Netflix's "The Crown") stars as Sawyer Valentini, a businesswoman who is still troubled by the residual effect of a stalking. Early in the film, Sawyer voices that she has had specific suicidal thoughts to a therapist. The next thing she knows, she is in a mental institution, stripped of her phone and personal belongings and forced to stay 24 hours for observation, having unknowingly consented to that in the forms she signed without reading.

The place is every bit as terrible as you'd fear it to be. On edge the whole time, Sawyer is threatened by a hostile fellow patient (Juno Temple) and ends up striking one of the staff. That gets her 24-hour stay extended to one week and even though she seems sane to us, there's nothing that anyone can do to get her released, not even her mom (Amy Irving), who flies in and pleads her case with the facility's icy administration.

Making matters even worse is the fact that Sawyer is convinced that the night shift staff member assigned to distribute pills to the patients is none other than David Strine (a sufficiently creepy Joshua Leonard, who you probably won't recognize from The Blair Witch Project), the ex who stalked her.

Is Sawyer insane? Or is this place actually the insurance-bilking scam that her friendly, cell phone-smuggling fellow patient ("Saturday Night Live" alum Jay Pharoah) claims it to be? Unsane keeps you guessing, until it doesn't, and it remains gripping all the way through.

In the hands of a lesser director, this would be a clear-cut second-rate thriller. But Soderbergh elevates the material instead of playing down to its level. It's very easy to imagine this as a terrible January movie, although it would have younger, more attractive actors filling the lead roles then. But the script from the unsacred duo of Jonathan Bernstein and James Greer (The Spy Next Door, Lindsay Lohan's Just My Luck, Larry the Cable Guy: Health Inspector) finds a trustworthy home in Soderbergh, who channels David Fincher in the execution while maintaining the guerilla spirit needed to pull this off on an iPhone.

Why shoot this on an iPhone? Certainly, it must have been cheaper than a digital camera. But it also seems like one of Soderbergh's self-challenges, like making an over four-hour movie about Che Guevara that few will ever see. Occasionally, you can notice the visuals have a phone video quality to them. Most of the time, though, you don't even notice. Soderbergh, who as usual handles cinematography under the pseudonym Peter Andrews, gives the film the flair and energy it needs, particularly arresting with discomfort in a trippy scene in which Sawyer reacts to an unprescribed pill slipped in with her dose.

Outside of the Ocean's trilogy, Soderbergh's box office record is spotty and I would be shocked if Unsane performs even on the modest level of movies like Logan Lucky and Side Effects.

Spoiler alert? Matt Damon has a short yet flavorful cameo as a personal security expert who advises Valentini in a post-stalking flashback.