Wet Brain, a new one-act drama by John J. Caswell Jr. that opened Tuesday at Playwrights Horizons, is one of those rare shows that is so ambitious, so promising that it actually deserves to be longer, to better tease out the trajectories of its very dysfunctional Phoenix family. Middle son Ricky (Arturo Luís Soria), a thirtysomething New York City gay guy with a job in finance and a boyfriend he pretends hasn’t dumped him, arrives home after years away – ostensibly to help his siblings with the care of his ailing father (Julio Monge), an auto body shop owner ensnared in the bowels of mental illness and alcoholism. The fact that he wasn’t exactly a role model for fatherhood makes Ricky’s return all the more fraught: What filial obligation do you owe to a family that drove you far from home, and for good reason?

Younger sister Angelina (Cici Fernández) is exhausted by the responsibility of looking after dad despite her nursing degree; plus, she’s embarked on a not-so-secret romance with a woman she admits reminds her of their mom (Florencia Lozano), who died tragically when they were still young. Older brother Ron (Frankie J. Alvarez), the classic older-brother bully, is in over his head trying to manage the family business while also showing early signs of following his dad’s tragic path.

Caswell does a remarkable job of depicting the dynamic of these siblings, both intimate and awkward, supportive one moment and then childishly vituperative the next. (As Ron pointedly tells Ricky at one point, “I was homophobic way before you turned gay and I’m supposed to change?”) And the backstory is ladled out gradually, in a way that feels natural and still maintains our interest.

But Caswell also has another agenda – seeking to dramatize the effects of severe mental illness, in this case Wernicke’s encephalopathy and Korsakoff’s psychosis, exacerbated by alcohol addiction. We see Joe shuffle menacingly about his rundown one-story house (impressively designed by Kate Noll), mumbling inarticulately, vomiting onto his clothing and occasionally standing stock still, transfixed, and pointing to a light in the sky. We also see him wield a knife to extract something from his own stomach, which sends him to the hospital and further frustrates the siblings’ efforts to establish some control over his treatment.

Director Justin Wills, working with lighting designer Cha See as well as sound designers Tei Blow and John Gasper, attempts to dramatize Joe’s hallucinations – highlighting the otherworldly sensation of seeing and hearing things that nobody else can. But these tableaux are held at a distance, and never let us really comprehend Joe’s own experience of them — at least until late in the 105-minute show.

Near the end of the play, we enter one of Joe’s hallucinations in a stunning coup de theatre moment that is visually captivating but dramatically rather hollow. Granted, we get more of the family backstory that Caswell has been withholding — but it comes in a wild delusion-ex-machina that leaves us questioning the veracity of all the secrets that are being revealed. We also get surprisingly little insight into the (perhaps unknowable) experience of mental illness or the internal logic of those in its throes, except that it seems scary and unappealing, like a carnival ride from hell.

It’s a memorable scene, to be sure, but it’s also a cheat. With a little more time, with a few added scenes, Caswell might have been able to tie up his narrative loose ends in way that didn’t depend so much on over-the-top theatricality. Reality can be dramatic enough.