A YA Romantic Heroine with OCD

How’s this for a swoon-worthy romantic moment? Aza (Isabela Merced), darkly beautiful and shy, find herself alone with Davis (Felix Mallard), a rich-kid dreamboat, at his family’s woodside mansion. They’re having a gentle conversation; the sparks are flying. As the music swells, you feel the time arrive for them to kiss. At which point we hear Aza’s worried voice on the soundtrack saying, “You’ll get his bacteria in your mouth. His bacteria will make you sick.” Or as she puts it a little later to her psychiatrist (Poorna Jagannathan), “How can I have a boyfriend if I hate the idea of kissing him?”

Obsessive-compulsive disorder can take many forms, and in “Turtles All the Way Down,” based on the hugely popular young-adult novel by John Green (“The Fault in Our Stars”), it takes a rather classic one: Aza spends her entire existence terrified of germs — of contamination and infection. She has a callus on the inside of her middle finger, one that’s been there for years. Every day, she tends it, provokes it, cultivates it; when her OCD gets intense enough, she’ll prod it until it bleeds. That callus is the focal point of her anxiety. When she gives it a fresh Band-Aid, it’s like a ritual that lets her think she’s purged the poison. At the moments when her mania is out of control, she tries to “kill the infection” by consuming hand sanitizer.

Yet Aza’s disorder isn’t merely a fear of contamination. As she explains to us, it has an existential dimension. Aza believe that because she’s just an “organism,” with a body full of microbes, all of which have the potential to contaminate her, her self isn’t really under her control. So in a way, she has no self. The demon of OCD feels like it has a more controlling identity than she does.

That may sound like a rather heavy burden to anchor a teenage love story. But young-adult fiction is full of tragic illness, terrible accidents, and other catastrophes. So why not OCD? There are times when “Turtles All the Way Down” is like a teen horror movie. (Aza’s voiceover monologues are accompanied by magnified images of microbes, which loom up like monsters.) Part of the appeal of YA fiction is that it takes ordinary adolescent anxiety and uses over-the-top concrete terrors to blow it up to billboard size. You might say that Aza’s OCD is a metaphor for adolescence, as Aza tries to gain some sort of control over what’s happening inside her.

But you don’t need to read it that way, since OCD isn’t just a metaphor. It’s a syndrome that exists. “Turtles All the Way Down” makes Aza’s OCD just specific enough to draw us in, then deals with it in a way that’s just manipulative and sugar-coated enough to leave us thinking, “Yep, it’s another YA movie.” This one, directed by Hannah Marks, isn’t badly done, yet the film is as reassuringly formulaic in its use of OCD as Aza is ritualistic when it comes to dealing with it.

At heart, “Turtles All the Way Down” is a teen romantic fantasy about not having enough daddy love. Aza’s father died of natural causes when she was very young, and she connects with Davis, who she knew as a little kid, after his own billionaire father mysteriously disappears. As Davis, the Australian actor Felix Mallard comes on as a soft-edged preppie, with echoes of the young Leo, the young Robert Pattinson, the young James Spader minus the reticence. His Davis always has the perfect (empathetic) (witty) (non-mansplaining) thing to say.

That makes him a soothing suitor for Aza, who Isabala Merced invests with a forlorn agitation that’s convincing enough, but never too messy. The film’s perkiest performance is given by Cree Cicchino, who plays Aza’s lifelong friend, Daisy, a magenta-haired writer of “Star Wars” fan fiction. She seems, for a while, like a cookie-cutter supportive bestie, but when her real thoughts about Aza come out (how difficult it is to be friends with her), the film hits a note of something genuine and even a touch daring: the self-centeredness that can result from dealing with a mental disorder.

But if the film, for a moment, looks like it’s trying to be realistic about a problem this chronic, how can it have the happy ending a YA movie requires? “Turtles All the Way Down” has its own quirky one, in which Daisy predicts that Aza is going to have a nice life, and we see it played out as feel-good reality. That’s all a bit much, though it’s a fitting wrap-up for a movie that feels, by the end, like it wants to be called “Germs of Endearment.”