In 1999, Petey Shelburn finds himself on a mission to return and destroy a mysterious drum-playing toy monkey he discovered at an antiques shop. However, his attempts are thwarted when the monkey springs to life and inadvertently triggers a chain of chaotic events, resulting in the gruesome demise of the shop owner, who is fatally impaled by a harpoon gun. Following this tragic incident, Petey vanishes, leaving his wife, Lois, to take care of their identical twin sons, Hal and Bill.
As the years go by, the boys stumble upon the monkey while rummaging through their father's belongings in a closet. Unaware of the danger it brings, they wind its key during dinner one night, which inadvertently leads to their babysitter, Annie, being decapitated. The tension escalates, particularly when Bill begins to bully Hal, pushing him to another reckless decision. Hoping for revenge against his brother, Hal winds the monkey's key yet again, but this time it is their mother, Lois, who suffers a sudden aneurysm and passes away unexpectedly in front of Bill.
Overwhelmed by shame and guilt, Hal disposes of the monkey and he and Bill relocate to Maine to live with their Aunt Ida and Uncle Chip. Tragically, the monkey mysteriously resurfaces in their new home, leading Bill to realize its sinister powers. Despite Hal's protests, Bill winds the key once more, resulting in Uncle Chip being tragically trampled to death shortly thereafter. The brothers, now desperate, decide to entrap the monkey within its box and toss it down a well, hoping never to see it again.
Fast forward twenty-five years, and Hal finds himself estranged from both Bill and his own son Petey, whom he fears might fall victim to the monkey's wrath. Compounding Hal’s troubles, he learns that his ex-wife and her new spouse, Ted, plan to officially adopt Petey, which leaves Hal feeling further alienated. In an altered twist of fate, Aunt Ida meets her end in a freak accident, prompting Bill to alert Hal to suspicious happenings at Ida's house. He suspects that the monkey is back and someone has been winding its key.
On his arrival, Hal learns from real estate agent Barbara that a series of bizarre, fatal mishaps have been plaguing the town since Ida's untimely death. When a freak accident involving a falling shotgun results in Barbara's shocking demise, Hal realizes that the monkey is behind the chaos after all.
Arriving at Ida’s home, Hal discovers that Bill now possesses the monkey and has been harboring deep suspicion over their mother’s death. With a local named Ricky acting under Bill's command, the tension escalates as Hal refuses to let Petey wind the key. However, fear grips Hal when Ricky pressures Petey at gunpoint to retrieve the monkey. After a series of events lead to Ricky's demise by a swarm of wasps, Bill's obsessive quest to seek revenge against Hal intensifies.
As chaos ensues, both brothers find themselves facing a catastrophic situation. In a final act of desperation, Bill forces the monkey to play without winding the key, unleashing a wave of death upon the town, which tragically claims the lives of Hal's ex-wife and Ted. In the midst of the resulting destruction, Bill and Hal reconcile, grieving over their mother and mending their fractured relationship. However, just as they seem to reclaim their bond, the monkey strikes again, leading to Bill's shocking decapitation by a bowling ball inscribed with Lois' name.
Now, amidst the ruins of their town, Hal and Petey confront their destiny as the monkey's avoidant owners, vowing to prevent the key from ever being wound again. As they drive through the devastation, a spectral figure—a pale, black-eyed man on horseback—passes them, symbolizing the inevitable consequences of their choices. Faced with their newfound fate, Hal expresses his desire to reforge his connection with Petey by suggesting they go dancing, something Lois dearly loved. After a moment of hesitation, Petey agrees, setting the stage for their uncertain future.
