Director Steven Soderbergh has shot an entire movie — a nerve-wracking, gripping, artistically composed thriller — using an iPhone 7 Plus. And because Soderbergh is an artist, he not only made a good movie, but he even made the case for using an iPhone in this way.

Obviously, you wouldn’t shoot “Gravity” with it, but the iPhone lends a certain roughness to the film, which is appropriate here, as well as a kind of hyper-reality. Long shots are a bit fuzzy, but close-ups are intense and unflattering in the best way. Every line on every face is right there. There’s no gentleness about the image, and so, you feel as though you’re seeing people raw.

“Unsane” should go down as an important movie for English actress Claire Foy, one that reveals a new aspect of her range. She is best known, of course, for starring as Elizabeth II in two seasons of Netflix’s “The Crown.” But here, she is hard driving and neurotic, with a flat American accent that is so perfect, one might almost suspect she’s English and faking it.

Sawyer (Foy) has a good job at a bank in Philadelphia and seems in command of herself. But in fact, she is emotionally rattled, having moved from her hometown in order to shake off a dangerous stalker. The trauma from the stalker situation has made it hard for her to relax, and she goes to a psychiatrist and is tricked into committing herself into an insane asylum.

“Unsane” is Soderbergh in his best mode. As in “Haywire” and “Side Effects,” he takes what easily might have been a lowbrow genre entry and realizes it so completely that he turns it into something extraordinary. He’s not a snob. He doesn’t run from the thriller’s gut-level appeal, and he doesn’t lard the action with intellectual embellishments. He takes every visceral shock and terror in Jonathan Bernstein and James Greer’s screenplay and figures out how to make the experience even more unbearable.

As people who have been there and back can tell you, the worst thing about being locked up in a mental sanitarium is that no one believes a word you say. If you say, “I’m not crazy,” they don’t listen. If you scream it, they count it as proof you’re gone. And if you scream it more than once, you get sedated into zombiehood. But imagine if the attendants, administrators and doctors know full well that you’re not mentally ill — but they don’t care. Imagine if strapping sane people into lumpy cots is just something they do. Routinely.

“Unsane” is shrewdly paced, with quiet interludes that allow the audience to breathe, followed by stretches that escalate right to the edge of a viewer’s endurance. As witness and victim, Claire Foy is an ideal audience surrogate, in that she seems to remain in a state of outrage for the entire film. Sawyer can’t make nice or go along to get along. She can only fight, a strategy that seems misguided at first, but then gradually begins to make sense.

Foy is terrific, and she gets strong support from Joshua Leonard, who is so creepy and pathetic as one of the attendants that it takes an act of will to remember that he’s acting.