Haley Whitehall, the star of Osborne High’s drama department, returns to an empty house and spots the egg-shaped kitchen timer lying inexplicably on the welcome mat at the front door. It’s odd, but she takes a nap and tries to ignore it. Later, an unidentified killer gruesomely stabs her to death with a knife.
By the next morning, Haley’s murder is all anybody can talk about in the small town of Osborne, Nebraska. Makani Young and her friends, fellow seniors Darby and Alex, all participate in the gossip, though Makani feels bad about sensationalizing Haley’s murder. Ollie Larsson, the loner boy Makani has feelings for but hasn’t talked to since they hooked up over the summer, comes to school with newly colored pink hair. Alex spreads rumors that Ollie killed Haley and used her blood to dye his hair.
When Ollie gets into an altercation with some members of the football team, Makani interferes. Later that night, she and Ollie admit to having feelings for each other and rekindle their romance. Makani’s Grandma Young discovers that Makani is texting a boy and tells Makani she has to meet Ollie before she’ll allow Makani to date him. Makani came to live with Grandma Young almost a year ago. She used to live in Hawaii, where she was involved in a mysterious incident that caused all her friends, including her best friend, Jasmine, to stop speaking with her.
When Makani brings Ollie home to meet Grandma Young, she’s surprised by how well they hit it off. Meanwhile, Darby and Alex are less supportive of the relationship. They eagerly ask Ollie for inside information about Haley’s murder, since Ollie’s brother and legal guardian, Chris, is a police officer with the Osborne Police Department. But once the mysterious killer strikes again, killing football star Matt Butler, Alex and Darby suspect that Ollie could be the culprit. Ollie’s parents died in a car wreck when he was in middle school. He went through a rough patch of drinking, doing drugs, and even having a sexual relationship with a woman in her 20s. He eventually tried to die by drowning himself in a river. Though Ollie sought therapy and recovered, rumors about his past behavior, his edgy appearance (he dresses all in black and has a lip ring), and his shy, withdrawn personality lead Alex and Darby (and other students) to view him suspiciously.
Makani clashes with her friends over their suspicions about Ollie. She knows that the boy who happily bonded with her grandmother over a mutual love of jigsaw puzzles doesn’t have it in him to be a killer. At the same, she agonizes over what her friends—and Ollie—would do if they found out about the questionable past she left behind in Hawaii.
That weekend, the killer strikes for a third time, murdering Rodrigo Morales, the school’s resident computer whiz. Rodrigo’s murder raises the tension around town. While Haley and Matt had been extremely popular, talented students, Rodrigo was totally normal. Suddenly, it seems like anybody could be the killer’s next victim. Principal Stanton cancels school. Everyone is terrified. Rodrigo’s murder devastates Alex, who had a crush on him. Makani goes to the Feed ‘N’ Seed store where Darby and Alex work to try to comfort Alex, but things are still tense between them.
When classes resume later in the week, a clinic in Omaha informs Grandma Young, who sleepwalks, that they can fit her in for a last-minute sleep study. Though she doesn’t want to leave Makani alone, she doesn’t want to give up the appointment either. She goes to the study but orders Makani to hang out with Darby until she returns from Omaha. Makani lies and brings Ollie home instead. They have sex. Afterward, Makani leaves Ollie upstairs to fetch a glass of water, and suddenly, the killer attacks her. Ollie runs downstairs to intervene. Grandma Young comes home during the attack and lunges at the killer, who stabs her in the abdomen. Realizing that he’s outnumbered, the killer flees the scene. However, the killer’s hood slipped off during the attack, revealing his identity: it’s David Ware, a totally unremarkable Osborne High School student. The paramedics take Grandma Young to the hospital. She’ll be okay but needs to stay in the hospital for a long time. Chris, Ollie’s brother, invites Makani to stay at the Larsson house while Grandma Young recovers.
By this point, Makani’s attack has made the local news. Darby and Alex come to the hospital to apologize for suspecting Ollie. Makani forgives them and finally comes clean to everyone about her traumatic past. She tells a story about a drunken hazing ritual for her old high school’s swim team. Things got out of hand, and Makani viciously cut off her ex-best friend Jasmine’s hair with a knife. The police charged Makani with assault, though they eventually dropped the charges. Makani’s already miserable, dismissive parents used the stress of the situation as an excuse to further ignore Makani and negotiate a divorce. When news of Makani’s assault on Jasmine spread, Makani’s classmates turned on her, and she received death threats on the internet. To this day, she feels that she deserves all the hate she received. She thinks she’ll always be a bad person for what she did to Jasmine. To Makani’s surprise, her friends respond to her story with compassion rather than judgment. They know that she’s a good person who just made a mistake.
After Makani shares her story, everyone decides it’s only a matter of time before David will strike again. They try to figure out how he’s selecting his victims. The friends decide David could be targeting people who have bullied others or who, like Makani, are hiding a dark secret. Ollie supports this theory, revealing that Rodrigo used to harass women on the internet. The friends decide that Zachary Loup could be David’s next target. Ollie forwards this information to Chris, and Alex and Darby head to the local memorial parade to warn Zachary.
At the memorial, Zachary harasses fellow classmate Katie Kurtzman after she acts uninterested in him. Makani and Ollie meet up with Darby and Alex. Zachary is stoned and initially unwilling to heed their warning, though he eventually complies. Meanwhile, Caleb Greeley, the privileged and ultra-religious son of Pastor Greeley, has misplaced his plume. Caleb is trumpet section leader for Osborne High’s marching band, which is about to perform at the memorial. Not wanting to make the band look bad, he runs inside Greeley’s Foods, the family supermarket he works at, to search for his bowtie. But David Ware is waiting for Caleb inside Greeley’s. David murders Caleb, mutilating his body and displaying it in a gruesome fashion.
Makani and Ollie leave the parade. They talk, and Ollie confides in Makani about his past. She’s touched, and they make their relationship official. They approach Greeley’s Foods, where Ollie’s car is parked. They find Caleb’s body, but not before David makes a narrow escape.
Meanwhile, Katie Kurtzman has returned home from the memorial to look after her younger siblings and work on college applications while her mother is at work. David Ware breaks into the house and murders Katie. After giving statements to the police, Makani and Ollie spend the night at the hospital with Grandma Young. The next morning, the local news reports that David Ware has hitched a ride back to town with an unsuspecting truck driver.
Later, on the eve of Halloween, Chris takes Makani and Ollie back to the Larsson house so the three of them can get some sleep. Then Chris returns to work. Ollie and Makani are supposed to return to the hospital, but Alex calls them having a panic attack. She’s supposed to play with the marching band for that day’s football game, but she’s terrified of David. Makani and Ollie head to the stadium to rescue Alex. They do so, then they meet up with Darby at the hospital parking lot. The friends revise their theory about David’s process for selecting victims. Now, they think David is targeting ambitious students. Rosemarie Holt has just won the barrel race at the state fair, and they suspect David might kill her next. They hop inside Ollie’s car and speed toward Rosemarie’s farm.
Makani and her friends reach Rosemarie’s farm to find David aiming his knife at Rosemarie’s throat. David leaves Rosemarie injured but mostly okay. David sprints toward the enormous Martin Family Corn Maze adjacent to the Holt farm, mortally wounding Alex in the process. Makani and her friends chase David into the corn maze, which is crawling with drunken college students and other tourists who have flocked to the maze due to the sensationalized coverage of the so-called “Osborne Slayer.” David stabs Darby and Ollie in the chest, and they both collapse to the ground. Makani assumes they’re both dead. She asks David how he could do this, and he says something cryptic about making his victims stay in Osborne forever while he gets to leave. Then David tries to stab Makani, but she escapes. He chases her out of the corn maze. Outside, Makani runs into the corn pit—a ball pit made of corn kernels. Before David can jump in after her, Darby appears behind him and pushes him in. He lands hard and doesn’t move.
Makani walks toward David and grabs his knife. As she stands over him, she realizes that David killed his classmates because he was bitter that they were ambitious enough to leave Osborne while he was fated to stay there forever. He killed people because he saw imprisonment at the distant penitentiary as his only way out. Makani realizes how pathetic David’s dream was. She also realizes that a person can never run away from their problems. They’ll be their same, troubled self wherever they go unless they find the strength to change themselves. Makani kills David, ending his reign of terror. The police arrive. Chris runs into the corn maze to find Ollie’s body while the other first responders shuffle Makani and Darby into an ambulance. Another police officer’s radio buzzes: it’s Chris, who reveals that Ollie is alive. Makani runs into the corn maze to find Chris and Ollie. She and Chris kneel on either side of Ollie, and everyone finally feels safe.