Against the strict orders of her father, teenager Amy Harper decides to join her new boyfriend, Buzz, along with her best friend Liz and Liz's irresponsible boyfriend, Richie, for a night at a gritty traveling fun fair. The excitement of the carnival pulls them in as they indulge in smoking marijuana, sneak peeks at a 21-and-over strip show, and heckle fortune teller Madame Zena while exploring the bizarre freaks-of-nature exhibit and a rather low-quality magic show. The night takes a darker turn when Amy's younger brother, Joey, sneaks out to join them, unaware of the horrors awaiting the group.
When Richie challenges them to spend the night in "The Funhouse," a shadowy dark ride, the teens decide to take the plunge. After the carnival closes, they carefully sneak back into the fairgrounds and settle down inside the ride. To their shock, they witness an unsettling scene where a man in a Frankenstein suit, the ride assistant, engages with Madame Zena, only for things to take a violent turn as he strangles her to death over a financial dispute.
In their panic to escape, the group finds themselves trapped, leading to a betrayal when Richie steals money from the strongbox related to Zena's fee. The fun fair's barker, Conrad Straker, soon discovers Zena's fate and, believing his adopted son, Gunther, is responsible for the theft, he lashes out at him. It's then that Gunther's true identity is revealed—his horrifying deformed face with sharp teeth and glaring ruby-red eyes sends chills down the spine.
As tension escalates, Joey is outside the funhouse, desperately trying to comprehend the disappearance of his sister and her friends. However, he is soon captured by other carnival workers, leaving him helpless to inform his parents about the dangers lurking inside. Back within the funhouse, dread unfolds as the teenagers fall victim, one by one, to the wrath of Conrad and Gunther. Richie is first to perish, brutally strangled, followed by Liz, who tries to seduce Gunther before he retaliates violently.
In a nail-biting sequence, Amy and Buzz attempt to flee through an exit only to encounter a gun-wielding Conrad, who justifies his actions by claiming he is protecting Gunther from the authorities. A struggle ensues, and while Buzz manages to stab Conrad, he falls victim to Gunther's rage as well. With Amy left to face the terror alone, she navigates through a maze of horror until she confronts Gunther in a heart-stopping climactic showdown.
Using her wit and a crowbar for defense, Amy's struggle culminates in an electrifying moment when Gunther is caught in machinery, leading to a shocking conclusion. As dawn breaks, a weary but determined Amy emerges from the wreckage of the funhouse, haunted by the night’s events yet strong enough to walk away from the eerie carnival as workers dismantle the remnants of the attractions around her.
