As the timid salesman Elliot Brindle struggles to make ends meet, his life takes a troubling turn. With his job lost and debts piling up, he bears the weight of responsibility not only for himself but also for his mentally disabled brother Michael, his pregnant fiancée Shelby, and his abusive father Samson. With desperation looming large, a seemingly harmless phone call offers Elliot a glimmer of hope: $1,000 to eliminate a pesky fly that has been tormenting him. After confirming the payment online, he embarks on a harrowing journey filled with thirteen challenges that promise increasingly larger rewards.
As Elliot dives deeper into this enigmatic game, he unwittingly draws the attention of law enforcement. His initial challenges—causing a young girl to cry, igniting an act of arson, and conning a homeless man using a lavish ostrich—raise eyebrows among the police. This culminates in the involvement of Detective Chilcoat, who takes charge of the investigation after a child recognizes Elliot from wanted posters.
Concerned for Elliot as he becomes wrapped up in his secretive activities, Shelby presses him for answers, which leads him to hurriedly concoct a surprise. For his sixth challenge, Elliot is ordered to take the body of a man who committed suicide out for coffee. As the clock ticks down, panic sets in, compelling him to swipe a cup from a table of unsuspecting police officers. In a moment of desperation, Elliot cleverly files a complaint against them, leveraging the situation to his advantage, and astonishingly completing two challenges in the process.
Throughout this twisted adventure, Elliot's sanity and moral compass are severely tested. He continues to grapple with the blurred lines between right and wrong, all while maintaining a facade of normalcy in the face of Shelby's growing unease and Chilcoat's relentless pursuit. In a moment of recklessness, Elliot becomes aware that a misplaced library card could serve as evidence of his misdeeds if discovered.
Meanwhile, Chilcoat narrows his focus on Vogler, a conspiracy theorist who is increasingly disturbed by the game's underlying intentions. As Vogler spirals into paranoia, he urges Chilcoat to eliminate Elliot at the first chance, revealing a disquieting truth about the game's goal: to turn players into monstrous beings, seduced by the promise of immense wealth.
Elliot's next set of instructions leads him to a desolate motel where he's confronted by a childhood bully. The caller's chilling demand? Sever the man’s arm. Although initially resistant to violence, Elliot's resolve crumbles under his bully's taunts, culminating in a brutal act of vengeance against those who once tormented him, yielding two more completed challenges.
The chaotic climax heightens when law enforcement interrupts Elliot and Shelby at their rehearsal dinner, drawn to the revelation from Michael about disclosing himself to a young woman after his medication was cut off. To buy his brother time to escape, Elliot reluctantly agrees to the next challenge: to demolish the banquet hall while belting out The Internationale. Struggling with his lack of enthusiasm for such radical acts, Chilcoat suggests that Elliot turn himself in, revealing that another player has been concurrently committing atrocities, including pushing an elderly person down the stairs and setting fire to a shrine.
Elliot's hesitance fades away as he finds himself compelled to take a hostage to fulfill his next task. In a frantic escape, he inadvertently injures the hostage and leaves behind his cell phone. In a desperate measure, he seizes an elderly woman who serendipitously appears, convinced she has links to his upcoming challenge. However, her enigmatic smile fades, and Elliot realizes that he must set a stainless steel wire across a dangerous road, unknowingly placing a group of unsuspecting bikers in peril. Although he manages to disarm the trap just in time, tragedy strikes as a rival player’s setup results in the deaths of innocent cyclists. Disgusted and disillusioned, Elliot flings away the phone, vowing to exit the game.
Upon returning home, Elliot is stunned to find out that his brother Michael has also been playing. The final challenge looms as Michael reveals that Elliot must eliminate a family member. Elliot desperately tries to reason with him, but the painful truth emerges: winning the game came at a dreadful cost—the loss of their mother and a lifetime of torment. In a heart-wrenching turn of events, Samson takes his own life to spare his loved ones further suffering.
Yet, Michael remains undeterred in his quest to dispose of Elliot. A violent clash ensues as Elliot fights back in self-defense, suffering multiple stab wounds yet ultimately prevailing. However, he relinquishes the prize money when he intervenes in the game one final time by shooting Chilcoat, a mastermind orchestrating the chaos from behind the scenes. In a twist of fate, Shelby chooses not to continue in the nightmarish game, leaving Elliot feeling relieved and vindicated. As he lays exhausted, laughter escapes him—at last liberated from the dark clutches of his recent past.
