Mia recounts a pivotal day in her life that would forever define her family. The narrative unfolds with the return of her brother, Eugene, who, due to a non-speaking genetic disorder, communicates through actions rather than words. The day takes a dark turn when Eugene rushes home after a walk with their father, Adam, but Adam is conspicuously absent. Mia, anticipating Adam’s arrival, is alarmed by Eugene’s behavior, which includes pushing her to the ground before retreating to his room.
Eugene, who wears a perpetual smile, exhibits a challenging-to-interpret demeanor. Mia, expecting Adam to follow Eugene home, is perturbed when the footsteps she hears turn out to be a deliveryman, not her father. As the day progresses, Hannah (Mia’s mother) and John (Mia’s twin) return home. Mia notices blood on Eugene, prompting her to care for him while awaiting Adam’s return. However, the situation takes a dire turn when the police arrive, led by Detective Janus, revealing Eugene’s involvement in a traffic collision earlier that day.
As Mia grapples with the unfolding events, she becomes increasingly concerned about Adam’s disappearance. The search for him intensifies, and the discovery of a bag in the rapids containing a notebook with the initials “H.Q.” sparks further intrigue. Mia recalls intellectual conversations with her father about the “Happiness Quotient” and begins to piece together Adam’s experiments on children, aiming to understand the correlation between expectations and happiness.
The investigation takes unexpected turns as the police uncover Adam’s exchanges with a woman named Anjeli, who remains unreachable. A cryptic phone message suggests secrets that Adam may have been hiding from his family. An ATM withdrawal of $1000 and a video featuring a bloodied man resembling Adam deepen the mystery. Mia is torn between hoping her father is not involved in wrongdoing and the fear that he may have betrayed the family in unthinkable ways.
Detective Janus attempts to interview Eugene, despite the challenges posed by Eugene’s non-verbal condition. The situation escalates when Eugene, frustrated, makes a high-pitched shriek reminiscent of a traumatic event from his past. Janus, unfazed, takes Eugene to the police station. In parallel, Mia and John embark on a quest to find a specialist lawyer, Shannon, who can assist in Eugene’s case.
During their visit to Eugene’s school to find Shannon, they encounter parents who hint at a grim revelation – Adam’s possible positive cancer diagnosis. The shocking news adds another layer of complexity to the family’s ordeal. Shannon, on her way to the police station, becomes a crucial figure in the family’s pursuit of the truth.
A turning point occurs when the family discovers evidence suggesting Eugene’s ability to communicate. Anjeli, a speech therapist, had been conducting secret sessions with Eugene, challenging the family’s assumptions about his condition. The revelation brings both relief and confusion, as Mia grapples with the realization that her brother could communicate all along.
Despite this breakthrough, the family faces new challenges. Anjeli’s connection to Adam and Eugene raises suspicions, especially when they learn that Anjeli was supposed to meet them in the park but was hospitalized with COVID. Mia, torn between relief and uncertainty, ponders the possibility of Adam having an affair.
As the investigation unfolds, Eugene becomes a prime suspect in the eyes of the police. A video surfaces, showing Eugene attacking Adam and attempting to repel another person. The family is shown the video, leading to Eugene’s house arrest. Mia, John, and Hannah are compelled to confront the possibility that Eugene might be involved in Adam’s disappearance.
The narrative takes an unexpected turn as Anjeli recovers from COVID and agrees to interview Eugene. She reveals a letter from her partner, providing insights into Eugene’s frustrations and potential motives. The delayed disclosure of Eugene’s communication abilities adds further complexity to the family’s understanding of the situation.
Eugene, through a painstaking process, recounts the events of the day in question. His narrative diverges from the video’s depiction, presenting a more nuanced and emotionally charged account. Doubts persist, and the police challenge the credibility of Eugene’s testimony, citing the family’s prior exposure to the video.
Amidst the uncertainty, Mia’s ex, Vic, brings forward a video suggesting a confrontation between Adam and Eugene at a waterfall. The revelation opens up new possibilities and casts doubt on the narrative presented in the video. Mia, grappling with conflicting accounts, questions the authenticity of the information she has received.
The family, seeking to exonerate Eugene, embarks on a search for evidence. Anjeli’s recovery, coupled with the family’s persistence, leads to a breakthrough. Crucial pages discovered in Eugene’s dirty shorts support his account of the events. The police, now convinced of Eugene’s innocence, discredit him from the inquiry.
Despite this development, Adam remains missing, and the family confronts the lingering mysteries surrounding his disappearance. A hundred days later, during the Korean tradition of mourning, Mia reflects on the power of perspective in shaping narratives. The unresolved fate of Adam and the authenticity of Eugene’s account continue to haunt the family.
Hannah learns of Adam’s positive cancer test and the need for a second test, introducing new layers of complexity to the family’s understanding of Adam’s actions. Eugene’s emerging personality brings both solace and challenges as the family holds a ceremony at the waterfall where Adam was last seen.
As doubts persist, Mia faces a critical decision regarding Adam’s phone code. The potential for truth hangs in the balance as Mia, torn between seeking closure and confronting uncomfortable realities, deliberately chooses not to unlock the phone. The narrative concludes with an air of ambiguity, leaving the reader pondering the many facets of the story and the elusive nature of truth.