“Unsane” is pulp seen through an iPhone

★★★5 Rated R. 98 minutes.

Steven Soderbergh, who briefly retired from Hollywood after lamenting its timid small-mindedness, has shot his second post-hiatus film entirely on an iPhone.

“Unsane,” a pulpy psychological thriller, is an exercise in both genre and technology. It’s a B-movie iMovie. And it’s 98 minutes of proof that the laborious apparatus of filmmaking can be not only light on its feet, but fit snuggly inside your pocket.

Of course, not everyone has an “Unsane” on their smartphone. Some of us just have Words With Friends and a couple cute kid pictures. But Soderbergh (“Out of Sight,” “The Knick”), who serves as his own cinematographer under the pseudonym “Peter Andrews,” is a restless, protean filmmaker prone to experimentation and live-wire immediacy in his movies.

“Unsane,” made secretly over two weeks last June, is also unusually timely. In the script by Jonathan Bernstein and James Greer, Claire Foy, the breakout star of “The Crown,” plays a data analyst named Sawyer Valentini who has relocated from Boston to Pennsylvania to flee a stalker.

When Sawyer’s married boss calls her into his office to, wink wink, suggest they travel together to a weekend conference, she senses the danger and briskly returns to work. But when she attempts a one-nightstand with a Tinder date, she violently withdraws at their first embrace and locks herself in the bathroom.

Seeking the advice of a therapist, she speaks to someone at an anonymous behavioral centered. The meeting goes well, Sawyer thinks, but as she’s departing she’s asked to wait in the lobby. Later, a curt attendee comes to lead her down a drab, darkened hall, ushers her into a room, locks the door, and asks Sawyer to empty her bag, hand over her phone and disrobe.

Sawyer learns that her offhand acknowledgement of occasional suicidal thoughts has gotten her admitted for 24 hours, and that she’s unwittingly signed away her rights for that time. Her mother (Amy Irving) can do little to free her.

“Think of your cell phone as your enemy,” Sawyer is told by a stalker adviser, played by an uncredited Matt Damon, who instructs her on the perils of social media exposure. But Soderbergh’s movie is a testament to the power of one’s phone, and not just because it was shot with one.