In the charming town of Green Town, Illinois, two inquisitive friends, Will Halloway and Jim Nightshade, experience an unexpected journey after being released early from detention for the minor offense of "whispering in class." Will is a more reserved and contemplative soul while Jim embraces a free-spirited and slightly rebellious nature. Born just minutes apart on Halloween, these boys are neighbors, with Will living harmoniously with his devoted parents, Charles and his mother, while Jim is raised by a single mother after his father disappeared from the picture. Their mundane lives take a turn when they are introduced to a charismatic yet mysterious salesman named Tom Fury, who promises that his lightning rods can offer protection from an approaching storm.
As the boys engage with their day-to-day activities, they become increasingly enchanted by the arrival of Mr. Dark, a captivating figure whose carnival appears in their town overnight, inducing a dreamlike allure. The townsfolk seem hypnotized by the various attractions, including an unsettling bartender named Ed, whose missing limbs shockingly reappear in a mirror, and their teacher, Miss Foley, who desperately seeks to relive her youthful days. Intrigued by a grand carousel that stands off-limits to the public, Will and Jim approach it, only to be met with suspicion from Mr. Dark himself, setting a tone filled with foreboding.
Their adventure escalates as they witness Mr. Dark employing dark magic via the carousel on his frail assistant, Mr. Cooger, causing him to regress into his youth. Disturbed yet powerless, Will and Jim seek guidance from Miss Foley, only to find her entangled with Cooger, who is masquerading as her nephew. Forced to escape, they feel the weight of helplessness against Mr. Dark's malevolence.
As night envelops the town, tensions flare between the boys, ignited by Jim’s jealousy regarding Will's age. Their quarrel is disrupted when they witness Tom Fury being tormented by Mr. Dark, who is intent on extracting secrets about the impending storm. The sinister Dust Witch, an assistant to Mr. Dark, attempts to tempt Fury, who remains steadfast in his silence. Sensing the grave peril, Will and Jim flee, only to confront a horde of spiders as darkness prevails.
Fortuitously, the lightning rod gifted to Jim by Tom Fury earlier reveals its power, protecting them from the arachnid menace. As the night slowly dissipates, they ponder the enigmatic forces of Mr. Dark's carnival and the unsettling events that continues to unfold.
As dawn breaks, Mr. Dark, accompanied by townspeople transformed into carnival-goers, orchestrates a dark procession through the streets. Initially unaware of the ominous undertones, Will and Jim soon realize that this parade is essentially a manhunt targeted at them. In the meantime, Charles, feeling increasingly distressed, confronts Mr. Dark, using his wit to thwart the carnival's plans momentarily.
Seeking refuge within the local library, Charles reveals to Will and Jim that this is not their first encounter with the carnival, sharing the dark legacy of his own past, where his father faced Mr. Dark but ultimately fell short in the struggle.
Undeterred, Mr. Dark invades the library, offering a tempting bargain to Charles: give up your youth for the boys' release. Remaining resolute, Charles rejects the offer, only for Mr. Dark to render him unconscious and abduct Will and Jim.
Upon regaining consciousness, Charles finds himself before a violent storm, its ominous clouds looming over Green Town. He rushes to the carnival grounds, where he encounters Jim's mother, who is grappling with Mr. Dark's influence, albeit briefly.
Inside the hall of mirrors, Charles confronts Mr. Dark, who taunts him relentlessly about his age. Just when despair seems to claim victory, Will tentatively expresses his unwavering loyalty to his father, channeling this love into a powerful force that ultimately drives away the Dust Witch and liberates Fury.
As Mr. Dark strives to exploit the carousel's powers to fulfill his nefarious ambitions, Will and Charles intervene just in time, saving Jim as a powerful bolt of lightning strikes, shattering the carousel. With Mr. Dark's powers waning and his essence beginning to decay, the trio escapes from the now chaotic carnival, which spirals upward into a storm-laden sky.
Returning to the safety of their hometown, Will, Jim, and Charles revel in their hard-fought victory with a spontaneous dance, their movements synchronizing in triumphant joy as the threat of darkness finally fades away.
