A dear friend from my undergraduate days pursued a PhD in linguistics. She became enthralled by language acquisition among the neurodiverse, including those incapable of speech, caught in a cone of silence and staring out at a humanity that neglects them. She was the first confidante I emailed after I discovered that my oldest son — severely disabled by a genetic condition and nonverbal — could read and communicate. Her response: Are you shocked?

Mia Parkson, the confident biracial narrator of “Happiness Falls,” Angie Kim’s deliciously brainy new thriller, seems immune to shock. Raised in a suburb of Washington, she strives to tidy the messes that spill all around her. She’s got a plan for any problem that comes her way. Twenty years old, quarantined at home during the pandemic summer of 2020, she manages — and frequently manages to annoy — her family: her clueless twin, John; her mother, Hannah, a linguist who emigrated from Korea; her father, Adam, a goofy “Mr. Mom” devoted to 14-year-old Eugene, whose double diagnoses of autism and Angelman syndrome command the family’s movements, finances and intimacies.

Eugene vocalizes at a high pitch, what Mia calls “splaughing” — “a mix of singing, laughing, and playing violin spiccato (with a lightly bouncing bow).” Because of his Angelman syndrome, he smiles continually, a joyful boy on the outside, his inner life discernible only through his gaze. Adam coordinates his son’s physical and behavioral therapies, a bureaucratic sinkhole of paperwork, as anyone with a special-needs child can attest. (Kim’s debut, “Miracle Creek,” was also a literary thriller that addressed the complications of raising a special-needs child.)

As “Happiness Falls” opens, Adam has vanished while on a morning hike with Eugene at a nearby park. The boy sprints home, his clothes speckled with twigs and brush, causing a minor car accident on the way and then knocking down Mia on the lawn, from where she dazedly hears steps — which she presumes to be her father’s — on the gravel driveway. She drifts off to sleep in the gentle June sunlight, waking to the full-blown emergency of her missing father. Her instincts kick into overdrive: She maneuvers through her family’s mounting anxiety, mulling various theories, aided (and obstructed?) by a detective appropriately named Janus.

Language itself is on trial in this novel. It fails both Mia and her translator mother. It fails Detective Janus. Science is another culprit; the same glitches in our DNA can produce disparate phenotypes. Kim layers her themes as precisely as a torte chef; her sentences burst with flavor. Mia opines on selective perception, “a type of confirmation bias. ... You expect x to happen, something happens that’s consistent with x (although also with y and z), so you decide x has happened. I expected Dad to come home with Eugene, so when I heard footsteps on the driveway five minutes after Eugene got home, I thought, Dad.”

The mystery deepens. The detective retrieves Adam’s knapsack from the park’s rapids: Inside are his cellphone, a manila folder, and an unfamiliar notebook “bloated with water” and titled “HQ,” for “Happiness Quotient.” Adam may have been using the family for his own investigation into personal satisfaction (or, in his case, a lack thereof). Mia discerns her father’s struggles, physical as well as mental; that Kim portrays Adam’s silent suffering with candor and compassion is itself an audacious choice. Eugene’s speech therapist, Anjeli (her name a play on his syndrome), may be complicit — while working closely with Adam on a breakthrough with Eugene, did a romance spark? A video suggests something major occurred just before Adam disappeared, something he kept from his wife: Together, Adam and Anjeli had pried open Eugene’s interior cell. Mia, John and Hannah watch the video, transfixed, their world gutted. “It was painfully slow and limited, but he did it, spelling words by poking letter after letter. No one was holding up his arm or touching him in any way ... Anjeli didn’t seem to be doing anything but holding the letter board in front of him, still and unmoving — a human easel.”

As authorities interrogate the family, a brilliant lawyer arrives, armed with graphs and bullet points to negotiate the legal morass ahead. (Local governments often intervene aggressively on behalf of disabled minors.) Mia’s analytical mind battles with her emotions. “Happiness Falls” milks that tension, channeling forensics even as Mia confesses Gen Z concerns in footnotes to the text that offer more leads than meet the eye. She questions whether a crime has been committed — perhaps her father wanted to escape his caregiver burdens? Eugene’s the sole witness, but he may be withholding information. When the siblings connect directly, he unleashes a wave of resentments. Eugene accuses Mia of intellectual hubris, considering him stupid. Mia reels: “I felt myself shrinking, the space between the atoms in my body narrowing, shortening, producing heat that encircled me.” In a single sentence Kim limns the profound impact a special-needs child has on a family: the compromises, challenges and compartmentalization.

In another writer’s hands this complicated plot might have flown apart, but Mia is an appealing guide through thickets of cognition. She’s cheeky, indefatigable and sensitive to how a bifurcated identity molds her. As she pieces together what happened on that park trail, the scaffolding of a thriller falls away, revealing happiness and sorrow, futility and hope, as more than a dance of neurons. We rarely see a novel this assured that centers on disability. “Happiness Falls” dares to unlock the enigma of love at the molecular level while serving up a page-turner.