In the year 1999, a troubled pilot named Petey Shelbourne (played by Adam Scott) stumbles into a pawn shop, covered in blood and clutching a sinister-looking toy monkey equipped with a drum. Desperately, he tries to convince the shop owner Shafin Karim that this Monkey is cursed, warning that whenever it beats its drum, dire consequences will follow. Just as he speaks, the Monkey begins to play its haunting tune, unleashing chaos when a mouse chews through a wire, triggering a harpoon gun that fatally injures the shop owner by impaling him. In a frantic attempt to end the horror, Petey grabs a flamethrower, intending to destroy the cursed artifact himself.
Meanwhile, the impact of Petey's actions reverberates through his family, as he deserts his wife, Lois, portrayed by Tatiana Maslany, and their twin sons, Hal and Bill, who are both played by Christian Convery. The narrative is guided by the perspective of Hal, now older and portrayed by [Theo James](Theo James), as he reflects on his complex relationship with his brother Bill, who often bullied him during their childhood. Lois, sharing a cynical view of life, contributes to a tense home environment.
Intrigued by their father's possessions, the boys discover The Monkey, which holds the ominous phrase “like life” on its box—a phrase they misinterpret. While at a hibachi restaurant with their babysitter Annie, portrayed by Danica Dreyer, the Monkey starts its dreadful music after Hal leaves it in the car. In an unfortunate turn of events, the chef, distracted by his flirtation with Annie, accidentally beheads her with his spatula, marking one of the first tragedies attributed to The Monkey.
At school, Bill's bullying exacerbates Hal's insecurities, leading Hal to consider using The Monkey against him. Hal turns the key in the Monkey's back and, after another haunting melody, Lois tragically collapses due to what’s later described by Adult Hal as a “boomerang aneurysm.” This catastrophic event shatters the family further, as Bill struggles to comprehend their mother's death during her funeral. In a desperate bid to sever ties with the curse, Hal attempts to destroy The Monkey.
The brothers are sent to live with their Uncle Chip (portrayed by Osgood Perkins) and Aunt Ida (played by Sarah Levy). Yet, The Monkey mysteriously reappears, heralding more misfortune as Uncle Chip is tragically killed during a camping trip, stomped by a herd of wild horses. In a moment of fear, the boys decide to dispose of The Monkey by casting it down a well, hopeful that it will no longer bring harm.
Fast forward 25 years, and Hal now leads a lonely life, haunted by his past choices. He has a son, Petey, played by Colin O’Brien, whom he only sees once a year, fearing he might end up like his mother. When Hal goes to collect Petey for their annual visit, he learns from his ex-wife, played by Laura Mennell, and her new partner — a so-called “leading expert,” Ted Hammerman, embodied by Elijah Wood — that Ted is adopting Petey and suggests Hal step back from his son's life.
In a grim twist, Aunt Ida faces her own misfortune when she accidentally falls through her basement stairs, only to be horrifically injured. As she tries to treat herself, she foolishly checks the stove, leading to a catastrophic fire that results in her demise. Meanwhile, the cursed Monkey resurfaces when it is discovered by a quiet boy named Ricky, the son of a local cop.
On their way to a horror-themed amusement park, Hal takes Petey to a motel. While there, Petey, curious about family history, presses Hal about his brother, prompting a lie that he has none. As evening falls, Hal receives a call from Bill, who reveals the Monkey's return and insists Hal must return to settle Aunt Ida's estate. A series of horrific accidents unfold, confirming that The Monkey has indeed been reactivated.
Upon their visit to Aunt Ida's house, they encounter a perky real estate agent named Barbara, played by Tess Degenstein, who divulges the dark events surrounding the house. An accident occurs as a shotgun falls from a closet and blasts Barbara, pushing Hal deeper into the madness surrounding him.
Unbeknownst to Hal, Bill has taken matters further, involving Ricky to get his hands on The Monkey, hoping to use it to kill Hal, avenging their mother’s death. Amid a web of chaos, as The Monkey spins its dreadful tune, shocking deaths occur, including Hal’s coworker Dwayne, who chokes tragically on his vape pen, showcasing the inescapable grasp of fate linked to the cursed toy.
In a climactic turn of events, Ricky’s clumsy mishap leads to swarming wasps attacking him after Petey activates The Monkey. Hal rushes to confront Bill, who in a furious outburst demands The Monkey kill Hal. The inevitabilities of consequence unfold as calamity strikes, resulting in a series of catastrophic events leading to a plane crash that devastates the town.
Eventually, as bodies litter the streets and devastation reigns, Hal reaches out to Bill, attempting reconciliation. However, in a final twist of irony, The Monkey triggers one last deadly event, leading to Bill’s demise. Hal and Petey drive off quietly, bearing witness to the wreckage of their past.
As they traverse the town filled with devastation, they encounter the eerie sight of a pale man on a white horse, symbolizing death itself, inviting Petey to join him in a dance – a nod to the shadow of Lois. With the echoes of destruction behind them, they find themselves passing by a bus full of cheerleaders, abruptly struck down, leaving behind a haunting reminder of the chaos unleashed by the cursed Monkey. After the credits roll, there’s a tantalizing teaser for Osgood Perkins’ next eerie venture, "Keeper."
