On Wednesday the 11th, four teenagers are brutally murdered by a serial killer known as the Blissfield Butcher (Vince Vaughn), who was previously thought only to be an urban legend. During his murder spree, the Butcher steals a dagger known as EL DOLO (actual name unconfirmed) from a collection of ancient artifacts kept in the teenager's house where the murders took place.The next day, high school student Millie Kessler (Kathryn Newton) suffers bullying from various students, including cheerleader Ryler (Melissa Collazo), and teacher, Mr. Fletcher (Alan Ruck). Millie finds solace in her friends, Nyla (Celeste O'Connor) and Josh (Misha Osherovich), who is gay, and her crush, Booker (Uriah Shelton). Josh tries to convince Millie that she's "a piece," while Nyla fears that Millie is so concerned with taking care of her mother that she fails to take care of herself. Millie lives with her older sister, police office Charlene "Char" Kessler (Dana Drori), and alcoholic mother, Paula Kessler (Katie Finneran). Her father died one year ago.Despite the emergency alert of the butchered teenagers, Blissfield Valley High School maintains its plans to host a homecoming dance on Friday the 13th. Against the protests of Nyla and Josh, Millie plans to skip the homecoming dance and spend the night at a theater play with her mother instead.That night, the night of Thursday the 12th, Millie works as the high school mascot, a beaver, for the homecoming football game. She declines a ride home from Nyla and Josh and instead waits for her mother to pick her up. Unbeknownst to Millie, her mother has suffered another alcoholic fit and is passed out on the couch. Night comes and the high school crowd disperses, leaving Millie alone, still waiting for her mother. As she waits, she spots the Blissfield Butcher across the street. As the Butcher begins to approach, Millie runs underneath the stands of the football stadium to hide. As she attempts to escape, she is tackled by the Butcher. The Butcher stabs Millie in the left shoulder with EL DOLO, causing an identical wound to appear on his left shoulder. Charlene arrives and shoots her gun in the air to scare off the Butcher. The police keep EL DOLO as evidence.The next day, Friday the 13th, the Butcher awakes in the body of Millie in her bedroom. Meanwhile, Millie awakes in the body of the Butcher on a filthy mattress in an abandoned shack. In Millie's body, the Butcher attempts to stab an oblivious Charlene in the back with a kitchen knife but is intervened by Millie's oblivious mother. "Millie's" strange behavior is attributed to shock from her attack. The Butcher decides to go to school in Millie's body to find more teenagers to kill.The Butcher gives Millie's body an updated appearance, wearing a high ponytail and red lipstick and borrowing a red leather jacket from Charlene. This new appearance gives "Millie" attention from her previous bullies, including the football team. While at school, the Butcher ignores Nyla and Josh. He kills Ryler by locking her in a cryo-tank. As he exits the murder scene, he is at first shocked by the appearance of two police officers walking down the hall but quickly realizes that they do not suspect him since he is in the body of a teenage girl.The real Millie, trapped in the Butcher's body, sneaks into school to try to find her friends. Sketch drawings of the Butcher is all over the news, so Millie must keep a low profile to avoid being recognized. She also struggles to adjust to her new body's newfound strength and size. Millie finds Nyla and Josh, who recognize her as the Butcher, and begins chasing them in an attempt to explain her situation. After a mild scuffle in the kitchen of the high school, Millie is able to disarm her friends long enough for her to do the school mascot's dance routine and their friendship handshake, proving that she is Millie after all. Nyla and Josh ask their Spanish teacher to translate an article about EL DOLO, which reveals that Millie has until midnight to stab the Butcher with EL DOLO or else the body switch will be permanent.Millie, Nyla, and Josh realize that the Butcher is "a wolf in sheep's clothing" and set out to find him. When they find the Butcher in the school hallways, he cries out for help, drawing the attention of police officers, students, and teachers in the school. Millie, Nyla, and Josh escape in Josh's car. They are chased by Charlene, who is on duty. They escape to the Discount Bonanza store, where Millie's mother works. Millie hides in a changing room, where she and her mother have a meaningful conversation through the changing room door. The exchange ends with Paula asking "the Butcher" on a date, to which Millie declines politely and says "he" is a married man. The Butcher kills Mr. Fletcher.Nyla and Josh find an Aaron Rodgers mask for Millie to don to avoid being recognized. As they leave the store, Josh discovers that the Butcher is at an arcade with a few members of the football team. The Butcher lures Booker into a laser tag arena. Millie, Nyla, and Josh arrive to save Booker. Millie knocks the Butcher and Booker unconscious.At Josh's house, Millie ties the Butcher to a chair. She and Nyla try to explain the situation to Booker, but he remains unconvinced until Millie recites a love poem that she anonymously slipped into his locker a few weeks earlier. Josh stays at his house with the Butcher while Millie, Nyla, and Booker go to the police station to obtain EL DOLO.Nyla enters the police station and tricks Charlene, the only police officer at the station, to look for the Butcher outside. While Charlene is outside, Nyla steals her keys and obtains EL DOLO from the evidence locker. Millie and Booker wait in the car and have a meaningful conversation in which Booker reveals he's liked Millie "from day one." Millie confides to him the newfound strength she feels being in the Butcher's body and not being bullied, but Booker tells her that strength is in your heart and your mind, not your size, and that Millie is really a lot stronger than she thinks she is.Josh's mother arrives home early to find "Millie" tied up in their kitchen. Josh attempts to convince his mother that he is actually straight and this is just part of sexual role playing. However, the Butcher slips out of the ropes tied around his wrists and chases Josh and his mother down the hallway with a kitchen knife.At the police station, Charlene catches Nyla stealing EL DOLO. Millie sees the Butcher run into the police station and follows in after him. Charlene attempts to put Millie in a jail cell, but Millie overpowers her and apologetically locks her in the cell instead. The Butcher escapes in a police car and almost runs over Josh.At the homecoming dance, the Butcher kills four football players. Millie, Nyla, Josh, and Booker drive back to the homecoming dance in search of the Butcher. Booker sets an alarm on Millie's Apple watch for midnight. They find the Butcher, and Nyla and Josh tackle him and hold him down while Booker staves off the police. As Millie prepares to stab him, her alarm beeps, indicating that it's too late and the curse is permanent. However, Josh notices that the bell hasn't rung yet, and Millie remembers when Booker gave her advice to set her clock five minutes late to prevent being late to classes. Millie realizes it is actually 11:55 PM and stabs the Butcher. They switch bodies just as the police discover the Butcher (now in his own body) and shoot him down.While in an ambulance, the Butcher removes his fingertip pulse oximeter to fake his death. Meanwhile, Charlene and Paula go to sleep. Millie discovers the back door open and is attacked by the Butcher. The Butcher torments her. He says that after being in her body, he understands why she's so anxious and weak. Charlene appears and orders the Butcher to freeze. She attempts to fire her gun, but the Butcher reveals he removed all its bullets. Suddenly, Paula smashes a wine bottle the Butcher's head. He quickly overpowers all three women, throwing Charlene into the TV, punching Paula in the face, and head-butting Millie. Millie admits that she also learned something from being in his body-that "having balls sucks." She kicks the Butcher in the balls and stabs him in the chest with a wooden post. She looks down at the dying Butcher and utters, "I am a f---ing piece."