In the small town of Vernon, on October 27, 29, and 31, 1987, three teenage girls named Tiffany Clark, Marisa Song, and Heather Hernandez were respectively killed by the Sweet 16 Killer, each having been stabbed 16 times on the nights of their 16th birthdays.In the present day, teenager Jamie Hughes goes to a concert with her friend Amelia on Halloween night while her mother, Pam, stays at home. Pam used to be friends with the three victims of the Sweet 16 Killer, and while handing out candy, she is attacked by the killer, who stabs her to death. As Jamie grieves Pam's death, she helps Amelia put the finishing touches on a time machine she created for a school project and is approached by reporter Chris Dubasage, who tells her that Pam received a note from the killer that she kept secret: "You're next, one day."At night, Jamie is chased by the killer and hides in the time machine, which activates and sends her back to 1987. She realizes that if she stops the Sweet 16 Killer's original spree, she can save Pam's life. Jamie, masquerading as an exchange student from Canada, discovers that Pam and her friends were despised by everyone in the town as they were bullies. Jamie manages to convince Amelia's mother, Lauren, and a teenager named Doug, her school's present-day principal, to help her. Although Jamie manages to infiltrate a party hosted by Tiffany, she is unable to stop her murder. Using the trauma to bond with Pam, she infiltrates their group and convinces them to leave for the weekend, although this backfires; they end up at the cabin where Marisa was killed in the original timeline. As Jamie works to protect Marisa, Heather is murdered instead, changing the timeline.On Halloween night, the group reconvenes at the amusement park while Lauren works on turning a popular attraction into a makeshift time machine for Jamie to travel forward. In the present, Chris helps Amelia fix their time machine as they realize the timeline is changing. The group lures the Sweet 16 Killer into a haunted house where he attacks them, only to be impaled by Kara, the town sheriff in the present, with a scythe. Doug is revealed as the killer, seeking revenge on the group for the death of his girlfriend Trish. Marisa reveals that she, Tiffany, and Heather got Trish drunk one night and let her drive home, with her dying in an accident; Pam, however, was not present. As Jamie wonders why Pam received the note, a second killer appears and slits Marisa's throat.The second killer chases after Jamie, killing Chris' father along the way. The two face off in the new time machine as it activates and the killer is revealed to be Chris from the present. Doug was the original killer, but Chris murdered Pam and forged the note in order to generate more content for his podcast. The two fight and Jamie kicks him into the spinning machine, killing him. Upon returning to the present day, she finds that Pam is still alive. Since Jamie's parents got together earlier than they had originally, she now has an older brother named Jamie and her name is now Colette.