Frank Murdoch, a middle-aged insurance salesman, finds himself drowning in disillusionment as he observes modern society spiraling towards chaos, heavily influenced by pop culture, television, the internet, and the incessant chatter of talk radio. His frustrations build up as he envisions ways to silence his noisy neighbors whose baby's wails only worsen his migraine and disturb his much-needed sleep. The following day, Frank faces an irritating inconvenience when his neighbor's flashy sports car blocks his aging vehicle. When he is asked to move it, he feels a surge of irritation at their audacity.
At home, Frank's ex-wife, Alison, has custody of their daughter, Ava, who has transformed into a entitled brat, further souring his outlook on life. After innocently obtaining a co-worker's address to send her flowers, he finds himself unexpectedly fired from his job. Things take a dire turn when a visit to his doctor reveals a devastating diagnosis: a terminal brain tumor.
That evening, as Frank sits in front of the television in a daze, he becomes captivated by a talent show called American Superstarz. On it, Steven Clark, an intellectually disabled contestant, endures merciless mockery from the judges despite his brave attempts at singing. As Frank's despair overwhelms him, he contemplates ending his own life. However, his attention shifts to a reality show featuring Chloe, a ridiculously spoiled teenager, igniting a disturbing epiphany. He steals his neighbor's car, drives to Chloe's school, and despite a failed attempt to blow up her vehicle, he ultimately succumbs to darkness and murders her, witnessing the act is fellow student Roxanne "Roxy" Harmon. Initially misinterpreting Frank's actions as lecherous, Roxy confronts him before unwittingly becoming a companion in his spiraling journey.
As their tumultuous relationship develops, Roxy challenges Frank to divert their anger towards Chloe's indifferent parents, who seem more upset about the financial ramifications of their daughter's demise rather than the emotional loss. When they arrive at Chloe's home, Frank executes Chloe's father with precise aim while Roxy brutally kills her mother. During their grim escapades, Roxy reveals her troubled background, a tale of addiction and abuse that resonates deeply with Frank, drawing him further into her destructive worldview.
Their violent journey lands them in a cinema screening a documentary about the Mỹ Lai Massacre, which is abruptly disrupted by unruly teenagers. In a fit of fury, Frank and Roxy open fire on the group, leaving only one survivor—a gesture for whom Frank expresses gratitude before eliminating yet another threat recording their chaos.
Their killing spree becomes a reckless tour de force as they target individuals who embody everything they resent in society: double-parkers, far-right protesters, and a well-known right-wing commentator. In an unexpected twist, Frank chooses to spare his ex-wife's new fiancé, deeming it excessively cruel to inflict suffering on him alongside his wife and selfish daughter. Roxy later proposes a drastic shift in lifestyle, planning to flee to France to live a peaceful life raising goats and making cheese, fantasizing about a legitimate existence.
Just as Frank begins to entertain this dream, the fragile illusion disintegrates when his doctor informs him that his MRI results were confused with another patient's. As they enjoy breakfast at a motel, their morning is ruined when a diner customer mistakenly identifies Roxy as an underage prostitute and assumes Frank is her pimp, igniting a series of emotions and revelations. Frank learns that Roxy's so-called chaotic life is rooted in a seemingly perfect middle-class upbringing, leading to feelings of wrath and betrayal. This culminates in a brutal confrontation when he kills the man who attempted to solicit Roxy's services.
Conflict arises as Roxy admits to deceiving him, claiming she sought an escape from the banality of her life. In a moment of frustration, Frank hands her the keys to the neighbor's car, walking away in the pickup truck of their latest victim, leaving Roxy with bitter reflections.
As Frank edges closer to desperation, he apprehensively acquires an assault rifle from a shady dealer. The unfolding news coverage reveals Roxy's return to her parents, inciting Frank's next move. He manages to infiltrate the American Superstarz studio, unleashing a horrific wave of violence that results in the death of multiple audience members and a judge, as he takes the studio hostage.
Amid the chaos, Roxy surprises everyone by reappearing on stage to apologize for her earlier lies. In a heated confrontation, Frank criticizes the treacherous state of contemporary American society, highlighting its moral decay. The notably invited guest, Steven, only fuels Frank's fury when he learns that his earlier suicide attempt was rooted in a longing for companionship and purpose, rather than malice.
With a softened demeanor, Frank turns to Roxy, calling her beautiful—a compliment he had refrained from offering previously due to her age. This fleeting kindness immediately gives way to violence as Frank and Roxy engage in a bloody massacre, targeting Steven, the judges, and the audience before meeting a tragic end at the hands of the police, who arrive just as the senseless carnage reaches its devastating conclusion.
