After four teenagers discuss urban legends of a serial killer known as the Blissfield Butcher, the Butcher breaks into the mansion they are in and promptly murders the group of teenagers before leaving with an ancient dagger known as La Dola.

The next day, bullied high school student Millie Kessler attends the Blissfield Valley High School homecoming football game, where she performs as the school mascot. As Millie waits for a ride home near the now-empty school, the Butcher attacks her. He stabs Millie in the shoulder with La Dola, causing an identical wound to instantly appear on his shoulder. Millie's older sister Char, a police officer, arrives and scares off the Butcher. The police collect La Dola as evidence and initiate a manhunt for the Butcher.

The following morning, the Butcher and Millie discover that they have switched bodies, and they both make their way to Blissfield Valley High School. At school, the Butcher, now posing as Millie, kills Millie's chief tormentor Ryler by locking her in a cryotherapy tank in the girl's locker room. He realizes his innocent appearance grants him immunity from suspicion and fatally slices woodshop teacher Mr. Bernardi, another one of Millie's tormentors, in half with a table saw. Millie, now in the Butcher's body, finds her best friends Nyla and Josh and proves her identity to them by performing the school mascot's dance routine and answering a series of personal questions. Nyla and Josh research La Dola and discover that Millie must stab the Butcher with the dagger by midnight or else the body switch will be permanent.

Later that afternoon, with the school's official homecoming cancelled, the Butcher suggests a new dance be held at an old mill that is actually his hiding place. The Butcher subsequently lures Millie's crush Booker into a monster mini golf course to kill him, but Millie, Nyla, and Josh arrive just in time to save him. Millie knocks both the Butcher and Booker unconscious, and she and her friends bring the two to Josh's house. After tying the Butcher to a chair, Millie and Nyla try to explain the situation to Booker, who remains unconvinced until Millie recites a love poem she anonymously wrote to him weeks earlier. Josh watches over the Butcher while Millie, Nyla, and Booker drive to the police station to obtain La Dola. Nyla tricks Char, the last officer in the station, into leaving so she can steal the dagger. Waiting outside in the car, Booker reveals that he has always liked Millie, and Millie divulges how she enjoys the newfound strength and confidence she feels while in the Butcher's body, and they kiss.

The Butcher eventually escapes Josh's house, and Char catches Nyla stealing La Dola. Still in the car, Millie sees the Butcher enter the police station and runs in after him, but Char tries to detain her, unaware of the situation. Millie overpowers Char and locks her in a jail cell while the Butcher escapes in a police car. At the Blissfield Valley High Homecoming dance out at the Butcher's mill, the Butcher kills three jocks who attempt to rape him, thinking he is Millie, as well as a fourth who attempted to assault Josh. As midnight approaches, Millie finds the Butcher and both Nyla and Josh hold him down while Booker staves off the police. Millie stabs the Butcher with La Dola and they switch back to their own bodies just in time, before the police shoot down the Butcher. Later, Millie and Booker reunite and they kiss again.