Steven Soderbergh’s new movie Unsane fits into a long line of “Who’s the crazy one here?” stories. Films like Alfred Hitchcock’s The Lady Vanishes, Otto Preminger’s Bunny Lake Is Missing, and modern equivalents like Flightplan and The Forgotten all hinge on protagonists being told they’re deluded, and that someone they clearly remember never actually existed. Unsane plays a similar game, with The Crown’s Claire Foy as a fragile, angry woman convinced that her stalker has somehow infiltrated the psychiatric ward where she’s been committed. For the first act or so, Soderbergh teases the audience with the possibilities. Is Foy’s character Sawyer actually being stalked, or are her minders right when they tell her she’s irrational, and can’t trust what she’s seeing?

Audiences may feel like they’re in the same position as Sawyer, though, being gaslighted over the huge gap between what they’ll see onscreen and what Soderbergh is telling them they’re seeing. Soderbergh shot the entirety of Unsane with the iPhone 7 Plus, and he’s been singing the camera’s praises in the press. “I’ve seen it 40 feet tall. It looks like velvet. This is a gamechanger to me,” he told Indiewire. “I look at this as potentially one of the most liberating experiences that I’ve ever had as a filmmaker.” He’s already shooting a second iPhone-only movie, an NBA drama called High Flying Bird.

Soderbergh isn’t the first director to shoot a theatrical feature entirely with iPhones, but he’s the most prominent and well-established filmmaker to try it to date. The 30-year industry vet has a long list of beautifully shot films under his belt, including Out of Sight, the Oceans 11 movies, The Informant!, and the Magic Mike films. He usually serves as his own cinematographer; he likes to experiment with form and style. He’s knowledgeable about camera tech and how to match the look of a film to its content.

But his latest film isn’t as convincing as his rapturous quotes about it. Much of Unsane takes place in that hospital, where Sawyer tries to convince the staff first that she’s perfectly fine and under control, and then that one of the orderlies, George (Joshua Leonard) is actually the predatory man who ruined her life. Her ward is a grimy, under-lit space, and the iPhone’s 4K video camera doesn’t handle low-light conditions particularly well. Faces disappear into fuzzy blurs, the colors trend toward a muddy yellowish-brown, and light sources burn out into blazes of distracting white. The Caucasian characters look blotchy and ill, and the black characters look like shadows. It’s easy to see how shooting on a phone was liberating in technical terms, especially for a filmmaker who is perpetually working on DIY models that will let him escape the Hollywood finance and marketing industry. But it’s hard to believe this was the best aesthetic choice for the film, especially for a small-scale human story where all the action comes from the characters’ encounters, and their facial expressions are so important to the action.

Narratively, Unsane is a harrowing and memorable movie. Screenwriters Jonathan Bernstein and James Greer drop viewers into what feels like season 2 of a running story: Sawyer has just moved to a new city, started a new job, and gotten a new home, all to evade a stalker who decided they were destined for each other, and has settled all his mad fantasies on an image of her that he’s created for himself. When she confesses her anxiety and fear to a counselor, she admits that she’s considered suicide. The counselor promptly hands her some paperwork to sign, and before Sawyer knows what’s happening, she’s locked in a room with a series of no-nonsense nurses, being ordered to strip and surrender all her belongings. The paperwork she didn’t bother to read has committed her for observation, in a closed ward where she’s surrounded by threatening and unnerving patients, and hospital employees who treat her like a misbehaving animal. She lashes out in frustration, but every blowup is taken as further indication that she needs to be institutionalized, and her situation gets progressively worse.

Stories like Unsane and its predecessors in the “Who’s crazy here?” subgenre are at their best when they openly play with the audience’s shifting understanding of what’s really going on. Films like Bunny Lake Is Missing draw the mystery out at length, making it seem possible that the protagonist really is delusional, and that she’s the only one not aware of her own break with reality. Unsane doesn’t tease out the thread as long as other films of this type, but the sheer strength of its character work compacts a lot of queasy horror into a small space. Sawyer’s rage over being threatened, caged, drugged, and restrained is understandable, but she also goes to a place of shrieking violence so quickly and completely that it’s easy to understand why her minders question her rationality. There’s a lot of empathy to go around in this story — it’s clear why Sawyer’s reactions are so extreme, and why she finds her situation so infuriating and untenable. It’s also clear why anyone watching her would think she’s a danger to herself and others. The filmmakers do a terrific job of building the tension in ways that invite viewers to wonder how well they’d comport themselves in Sawyer’s position, and whether they could find a way out of the room she’s condemned to.

Once the truth becomes clear, though, Unsane loses a lot of its energy as it tries to resolve a more familiar, conventional scenario that’s sometimes clumsily told. Sawyer goes through a major character shift that’s so abrupt, it isn’t entirely convincing. Foy gives an intense and intimidating performance, but it’s often hard to relate to her, given the extremity of her choices. The script unquestionably presents her as damaged and dysfunctional, for reasons that aren’t her fault — she’s dealing with extreme trauma as best she can. The decision to make her unlovable and sometimes frightening is daring, given the perpetual pressure to make female characters approachable and likeable. Her flaws and frailties are a major part of the story, and one of its most startling and thrilling ideas.

But the story calls for her to be ruthless about using other people for her own ends, usually with ugly and depressing fallout. In particular, she takes merciless advantage of Nate (Saturday Night Live’s Jay Pharoah), the one calm, sympathetic fellow inmate who attempts to relate to her. At times, only the extreme injustice of her situation keeps her compelling — that, and the outrageous behavior of the film’s antagonist.

Given all the grubby, grotesque emotions on display in Unsane, maybe it’s fully appropriate that the film itself so often looks grubby. The murky images onscreen are a match for what Sawyer is feeling, as she’s told she can’t trust her own perceptions or memories, and as everything she trusts to help her gradually fails her. The scattering of outdoor scenes in the film do bring realistic color and depth back into the story, and they speak better for the iPhone camera than the indoor shots.

But it’s harder to excuse the footage’s soft, blurry qualities, the way only the well-lit scenes really look like they’re fully in focus, and the sense that visual detail and depth is being lost in every indoor shot. For the most part, Unsane feels like one of the many early experiments in a technology that’s still developing, and hasn’t caught up with professional-grade movie cameras. It’s still more of a limited novelty than a cinematic revolution.