Taylor Russell in “Words on Bathroom Walls.” Jacob Yakob/Courtesy of LD Entertainment/Roadside Attractions

Imagine an adolescent boy-meets-girl movie – all those hormones, angst and meet-cuteness – but with a twist: He’s been diagnosed with treatment-resistant schizophrenia.

That is the strange, what-were-they-thinking premise of “Words on Bathroom Walls,” a teenage love story based on Julia Walton’s 2017 novel about a high school senior whose dreams of becoming a chef are threatened, along with almost everything else about his normal life, when he starts hearing voices and seeing people who aren’t there. The book has been described as surprisingly funny, and I can report that this is also true of the film, which has been adapted from the source material by screenwriter Nick Naveda (“Say You Will”) and filmmaker Thor Freudenthal, a director of mostly TV whose film résumé is notable for “Diary of a Wimpy Kid” and “Percy Jackson: Sea of Monsters.”

The humor is not necessarily a good thing.

Freudenthal’s experience puts him in good stead here, in a tale that, like “Wimpy Kid,” features both wryly self-aware narration – here delivered by the protagonist Adam (a likable Charlie Plummer) to his off-camera psychiatrist and, like the mythology-inspired “Sea of Monsters,” a bunch of entities that only he can see.

Lobo Sebastian, AnnaSophia Robb and Devon Bostick in “Words on Bathroom Walls.” Jacob Yakob/Courtesy of LD Entertainment/Roadside Attractions

In Adam’s case, that means a cloud of black smoke that resembles a Dementor from the Harry Potter films and that speaks to him in a rumbling baritone, as well as three hallucinatory personifications of his troubled mind: the calming, New Age-y Rebecca (AnnaSophia Robb); a baseball bat-toting bodyguard (Lobo Sebastian); and Joaquin (Devon Bostick), a sort of older-and-wiser romantic wingman who hangs out in boxer shorts and an open bathrobe.

It is, arguably, refreshing to see a story in which serious mental illness is not treated with the doom and gloom that normally accompanies such things. One scene features Adam seemingly encountering his potential future self when he spies a mentally ill man talking to himself on the bus. But that fleeting, scary moment is a rarity in a film whose lightness of touch and jaunty score can be jarring. The representations of paranoia and delusion that work on the page, when transplanted, quite literally, to the screen, feels, every now and again, out of kilter with not just our expectations but with the gravitas that the subject matter deserves.

It should be said that Plummer (so good in both “Lean on Pete” and “All the Money in the World”) does his best to make all this work, switching between the witty, sarcastic therapy sessions and his more painful school life. As the new kid at a Catholic school – where he has transferred after getting kicked out of his last one – he must also negotiate an experimental new drug therapy that quiets the voices but gives him tremors and affects his taste buds, along with a budding romance with his classmate and math tutor Maya (Taylor Russell).

Charlie Plummer in “Words on Bathroom Walls.” Jacob Yakob/Courtesy of LD Entertainment/Roadside Attractions

As so often happens in these things, Maya is from a struggling single-parent household on the wrong side of the tracks, and Adam is a product of the comfortable upper-middle class. Although Adam’s father has left, he’s been replaced with a new stepfather (Walton Goggins), whose grudging acceptance by Adam forms the trajectory of another subplot.

The parameters, in other words, are well familiar. This can makes the schizophrenia story line seem, well, slightly gimmicky: an awkward theme grafted onto an otherwise cliche narrative.

It’s certainly worth noting that grave mental illness doesn’t have to be a death sentence. But that point can come across, at the film’s more straight-faced moments, like sermonizing tacked onto a “message movie.” When “Words on Bathroom Walls” is at its sunniest and most blithe, the moral of the story feels a little more like a punchline than is appropriate.

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