In 1991, in the town of Kettle Springs, Missouri, a group of teenagers are partying near the old Baypen corn syrup factory, which was a symbol of the town's success until it burned down, apparently because of an arson attack. One of the teenagers, Jessica, decides to roam around the surrounding cornfield. One of the boys follows her, concerned, only to find her limping to him in the middle of the cornfield, fatally wounded. Upon her collapsing, the boy, shocked, fails to notice Frendo, the town's mascot and a symbol of a bygone era, lunging at him with a pitchfork.
Many years later, Quinn Maybrook and her father, Glenn, move into Kettle Springs, where Glenn is the new town doctor. Quinn and her father, whose relationship with her is fractured after the death of Quinn's mother, are greeted by their neighbor, Rust, who then walks her to school for her first day. Her first class goes awry, and her teacher, Mr. Vern, sends the entire class to detention after getting pranked by another student. In detention, Quinn manages to befriend her classmate Cole Hill and his group of friends, including Janet, a Queen Bee; Matt, a fitness buff; Ronnie, Matt's girlfriend; and Tucker, a nerd. Cole shows Quinn a video he and his group made on their YouTube channel about Frendo being a psycho clown living inside the old Baypen factory, which Cole insists was caused by faulty wiring despite the adults in town thinking otherwise. At a diner later that day, Sheriff Dunne, the town sheriff, tells her not to associate with Cole and his friends. Despite being told not to go, Quinn walks off to join Cole in going to an alcohol store, where Matt and Tucker stage a fight as Ronnie, Janet, and Cole sneak off with some alcohol bottles after Cole leaves a $20 note on the counter, leaving the shopkeeper, Otis, offended.
Later that night, Cole drives Quinn home after recording their first video with Quinn. Glenn angrily tells Cole to never see Quinn again after finding out they drank underage, as well as forbidding her from going to the annual Founders' Day festival the next day. The next morning, Quinn sneaks off to the festival anyway, joining Cole and his friends. The group sees nothing out of the ordinary except for Tucker's failure to arrive. At the festival, however, Cole and the gang watch as one of the ropes holding the Frendo float snaps, engulfing it in flames. Sheriff Dunne immediately accuses the group of sabotaging the festival despite evidence to the contrary. After being released, Quinn watches the video she and the others made to comfort herself, but her comfort turns to concern when she notices a second Frendo in one of the video's frames. Concerned, she sends the screenshot to Tucker. Tucker, upon seeing the picture, is immediately greeted with a notification from his doorbell. Seeing it as Frendo, Tucker grabs a Nerf gun and starts prowling the house for the intruder. Entering the guest room of his house, Tucker finds a large plastic bag on the floor, and subsequently notices a Baypen music box from in the closet. Cranking the handle, Tucker dismisses the springing Frendo as a childish joke before the intruder, dressed as Frendo, kills him with a sickle. The next day, Matt is killed by another Frendo while bench-pressing after he finds another Baypen music box, with Frendo attaching a hanging sawblade to the barbell before pushing it down on Matt's neck, killing him and sending his head into a cooler box.
That night, Quinn sneaks off with Cole after being grounded for getting arrested towards the old factory for a party being held. Cole introduces her to the DJ before dancing with her. Quinn takes Cole into a nearby shed before subsequently making out with him. Regretting it after, she runs outside to find Janet and Ronnie, but they are interrupted by their friend Ginger who collapses with a crossbow bolt in her back. Janet and Ronnie dismiss this as another one of Tucker and Matt's pranks. Matt's head is later thrown over to them, but Janet and Ronnie dismiss it as Matt going as far as creating a plaster cast of his own head. Quinn, notably concerned, checks Ginger's body for a pulse and upon not detecting a pulse, cause Janet and Ronnie to realize Ginger is actually dead.
Running to the barn, the door closes before they can get inside, cornering them with a crossbow-wielding Frendo. Running away from the barn towards the back, they run into Rust, who, carrying a shotgun, aims it and fires at the Frendo right behind them. Leading them to the shed, they run into Cole, who locks the shed doors as they are surrounded by multiple Frendos, who attack the door. As Quinn and the group escape via a sewer grate with Rust's shotgun only containing a single shell, Rust stays behind to set off a hastily-made bomb, killing all the Frendos surrounding the shed, with the goal of buying Quinn and the rest enough time to reach the Tillersons' house which has a landline available. Reaching a road, they call for a passing car only to find it being driven by Sheriff Dunne, who promptly refuses to listen to them and immediately arrests Cole for trespassing. Continuing on to the house, they are immediately surrounded by Frendos. Deciding to hide in the cornfield on the way, Quinn, Janet, and Ronnie are found by one of the Frendos, which immediately causes them to run, but not before Ronnie is killed. Quinn and Janet reach the Tillersons' house only to find a Frendo on the living room armchair. Thinking he's dead, Ronnie and Janet rush upstairs after hearing the sound of the landline ringing. They find out it is a rotary phone, which they cannot figure out how to use. Quinn, desperate, notices something on the bulletin board.
On the board, various news articles are pinned regarding Baypen's closure, along with pictures of groups of Frendos. Quinn immediately realizes not only is it a trap, but the 'dead' Frendo was actually meditating. The Frendo rushes at the two with a pitchfork, narrowly missing Quinn but impaling Janet to the ceiling in the process. Quinn pulls the pitchfork down and yanks it out of her before using it to kill the Frendo, causing Janet to bleed out and die just as the Frendo dies right next to her. Quinn immediately runs to Cole's truck in an effort to drive away, but unfortunately, she is unable to operate a manual transmission with clutch, which Cole's truck has. Soon after, Quinn is grabbed from behind by a Frendo, through the rear window.
Quinn later wakes up inside the old factory, slumped against a wall. Surrounded by Frendos, she is immediately attacked by a stun gun. Confronting the Frendos as to why they are killing people, the Frendos take off their masks to reveal themselves as the sheriff, Mr. Vern, Otis, and an elderly neighbor, Trudy. It is then revealed that they blame the teens for the town's troubles, especially Cole who is later revealed to be bound with a noose around his neck, with a pile of wood being the only thing keeping him alive. Arthur Hill, Cole's father and the town mayor, later disowns Cole and reveals himself as the leader of the Frendos, claiming it as defending their traditions. However, Sheriff Dunne and the Frendos, getting ready to execute Quinn, are interrupted by the arrival of Glenn, whose car kills several Frendos, knocks over the pile of wood, and effectively immobilizes itself in order to keep Cole alive. Rust later rescues Cole using a rifle, and teams up with Glenn to distract the Frendos as Quinn and Cole run in different paths in order to survive. Sheriff Dunne follows Quinn into a room full of Frendo costumes, and begins looking for Quinn. Quinn, disguised as a Frendo, takes off her mask, grabs a stun baton, and shoves it into Dunne's mouth, killing him. Meanwhile, after a wild goose chase through another room, Rust shoots Arthur. Running outside, they later find an unused Baypen Volkswagen Beetle as daybreak approaches. Safe, Glenn teaches her how to get the car, another manual with clutch, moving. Just as Quinn gets the car moving, they notice another Frendo in their way with a chainsaw. Quinn, goaded by her father, stomps on the gas and runs over the last Frendo, escaping.
One year later, Glenn is running for mayor, Quinn is leaving for college, and Cole and Rust have come to say goodbye. As Quinn drives off, she later finds a Baypen music box in the car. Unbothered, Quinn throws the box out the window.
