NOTE: This spoiler was submitted by Jeremy

Wednesday The 11th – Four teens – Ginny (Kelly Lamor Wilson), Phil (Magnus Diehl), Sandra (Emily Holder), and Tim (Zack Shires) – are talking about the notorious serial killer, The Blissfield Butcher (Vince Vaughn), who has apparently been committing crimes since the ’70s and has gone uncaught. Tim and Sandra go inside to get a bottle of wine while Ginny and Phil make out. Inside Ginny’s house is a mystical dagger called La Dola. Soon, Tim is left alone and is found by the Butcher, who jams a wine bottle down his throat and smashes it so the glass pierces through his throat. He then catches Sandra and bashes her head with a toilet seat. The Butcher then finds the other two and impales Phil’s head with a broken tennis racket before impaling Ginny on a rod in the living room. Her parents come home to discover her body, and the dagger is missing.

Thursday The 12th – Teenaged Millie Kessler (Kathryn Newton) wakes up for school. She lives with her mother Paula (Katie Finneran) and older sister Charlene (Dana Drori), who is a cop. It has been a year since Millie and Charlene’s father died, and the relationship between the three of them has been strained.

Millie goes to high school where she hangs with her best friends Josh (Misha Osherovich) and Nyla (Celeste O’Connor). The two are excited about the upcoming homecoming dance, but Millie says she is going to spend time with her mom. Although she has self-esteem issues, her friends assure her that she is “a piece”. Millie is antagonized by popular girl Ryler (Melissa Collazo) and her woodshop teacher Mr. Bernardi (Alan Ruck), who orders her to do a presentation he assigned her earlier than she was told, and she isn’t ready for it, so Bernardi just humiliates her in front of the class and her crush Booker (Uriah Shelton). The whole class is then alerted to the news of the murders. Ryler makes a phony video crying about how Ginny was her best friend, but Josh and Nyla point out that Ginny hated Ryler.

Later that night, Millie is acting as the mascot for the school’s football team, the Beavers, and she has someone throw soda at her. After the game, she waits for Paula to go pick her up, but when she calls, Paula is passed out drunk on the couch. Charlene calls Millie and offers to pick her up. As Millie waits, the lights around her go out. She is found by the Butcher and is chased onto the football field, where he takes out La Dola and raises it high. The ground beneath them briefly takes some kind of ancient form before the Butcher stabs Millie in the shoulder with it. Charlene arrives and shoots at the Butcher, who runs away. She takes her sister home, although Millie doesn’t seem like herself.

Friday The 13th – The Butcher wakes up in Millie’s body, moving around menacingly and bizarrely, but to Paula, it’s just what she thinks is a symptom of the trauma Millie felt from being stabbed. Meanwhile, Millie herself wakes up in the body of the Butcher down by the old mill. She freaks out and tries to go look for help. Unfortunately, everyone knows what the Butcher looks like due to a recent sketch being shown on the news, so anybody Millie comes across will panic and scream at the sight of him (or her).

Butcher goes to school as Millie after giving her a more alluring makeover that everyone takes notice of. When Ryler comes up to him, he goes away with her to the locker room in an…awkward attempt to get her alone to kill her. Millie then shows up at the school, where she tries to hide. Butcher and Ryler become aware of her presence, and Butcher takes Ryler to hide. He puts her in a freezer and locks her in, causing her to get frozen to death. When Millie finds her, Ryler’s body falls out and shatters. She then finds Josh and Nyla, who run away. She corners them in the kitchen and, after they stop throwing things at her, she proves that she is Millie by doing the Beavers’ cheerleading team dance routine. Josh and Nyla ask her questions only Millie would know, and they realize that it is their friend. After Millie mentions the dagger, Josh and Nyla go to his Spanish teacher, who knows about it and tells them that the switch becomes permanent after 24 hours, so they have roughly nine hours to get Millie back in her body.

Millie is in the bathroom where she sees the boys wrote crude things about her in the stalls. When one guy comes in making rude comments, she uses her size as the Butcher to intimidate the guy and make him wet his pants.

The Butcher finds Mr. Bernardi after he was being a jerk again. He puts on a protective coat and glasses before turning on the buzzsaw. Bernardi tries to fight Butcher until he jams a screwdriver in his neck, and then pushes his body through the saw to split him in half. Cops arrive at the school after they got word that the Butcher was spotted, which has also led to the cancellation of the homecoming dance. Butcher comes out of the woodshop just as Millie, Josh, and Nyla spot him. However, Butcher alerts the cops to their presence and has them chase Millie-Butcher and her friends. They hop in Josh’s car and drive away, but Charlene is on duty and pursues them. They manage to buy themselves time and run into the mall where they manage to get away from detection.

Josh and Nyla hide Millie in a changing room while they try to get her a disguise. Paula, who is working at the store, goes nearby to help, unaware that she is talking to her own daughter. They have a heartfelt conversation where they both discuss missing Millie’s dad, and when Paula feels that she is making a connection with this mystery man, Millie has to turn her down by saying she is married. Charlene then goes to get Paula before Josh and Nyla return with a rubber mask to get Millie out undetected.

Butcher is with a couple of jocks at an arcade who complain about the dance being canceled. As Millie, he suggests going to the old mill. One of the jocks, Brett (Ezra Sexton), puts his hand on Butcher-Millie’s butt, to which he tells him that his touch “makes this pussy drier than sandpaper”, which cuts deep for him. The Butcher sees Booker and tries to lure him to kill him, but Millie and her friends find them both and knock them unconscious.

The friends take Booker and the Butcher to Josh’s house. After some confusion, Millie convinces Booker who she is after reciting a poem that she left him in his locker a few weeks earlier. They keep Butcher tied up with Josh keeping an eye on him, while Millie goes with Nyla and Booker to the police station to retrieve the dagger from the evidence locker.

At the station, Nyla tells a suspicious Charlene that the Butcher kidnapped her and Josh, and has followed her here. This gets Charlene out while Nyla starts to search. Meanwhile, Booker talks to Millie in the car, admitting that he has always had a crush on her too. He admits to wanting to kiss her, and they do briefly until she realizes it would be better to do it when she is not in the body of a serial killer. Back at Josh’s house, his mom comes home to find “Millie” tied up, and when she tries to remove the binds, Butcher cuts himself free and tries to chase after Josh and his mom before running to the station. Millie and Booker go into the station just as Charlene has gone back inside to suspect Nyla, all while Butcher steals Charlene’s cruiser. They lock Charlene in a cell for a moment while they run to the old mill where the dance is taking place since they realize Butcher is going to try and kill as many people as possible there.

At the old mill, the dance is in effect, and Brett tries to bring Butcher-Millie with him to a private place. He and his buddies attempt a gang-rape, but Butcher smashes a bottle over one guy’s head and slashes another guy’s throat before grabbing a chainsaw and tearing into Brett’s groin with it. When the friends arrive, they see that they have roughly 13 minutes left before the switch becomes permanent. Booker sets an alarm on Millie’s watch as they go look for the Butcher. Another jock aggressively kisses Josh before the Butcher jams a hook into the guy’s eye. The cops show up and chase them as well, stopping Millie when she has the Butcher taken down. With help from her friends, they manage to bring the Butcher down, only to hear the alarm go off and realize they are too late. HOWEVER…Booker had mentioned that he likes to set his alarm five minutes ahead as an incentive, which means there is still time, and Millie jams La Dola into Butcher’s body to return to normal. The police then shoot the Butcher down, and Millie is thankful to be back with her friends. As she recovers, she and Booker share their first normal kiss, which Josh and Nyla cheer on.

The Butcher is being taken away in an ambulance. Paramedics try to keep him stable, but his heart rate appears to go down before flatlining completely.

Millie goes back home to Paula and Charlene. When it gets quiet, Millie suddenly hears a noise. The Butcher is still alive and begins to attack the three women. Charlene tries to shoot him, but he removed the bullets from her gun. After he incapacitates Paula and Charlene, Butcher taunts Millie for her weak body, until she points out a weakness in his own body by kicking him in the balls. This allows the women to overpower him before Millie grabs a broken table leg and impales him with it, finally killing him. As they stand over his body, Millie declares, “I am a fucking piece.”

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Teenager Millie Kessler sees herself as ordinary and has a strained relationship with her mother Paula and older sister Charlene after the death of her father a year earlier. Millie is attacked at school by notorious serial killer, The Blissfield Butcher, who stabs her with an ancient dagger called La Dola that causes them to switch bodies.

As Millie, the Butcher is able to commit murders without arousing suspicion. As the Butcher, Millie has to convince her best friends Josh and Nyla to help her turn back to normal before midnight, or else the switch will be irreversible. They also convince Millie's crush Booker as to who she is, and after they capture Butcher-Millie, they attempt to get the dagger from the police station. They succeed, but Butcher escapes and goes to the old town mill where the homecoming dance is taking place. The friends manage to catch the Butcher and help Millie return to her normal body before the police shoot the Butcher. She and Booker also share their first kiss.

The Butcher survives and goes to attack the Kessler women in their home. They manage to overpower him eventually before Millie impales him with a broken table leg, ending his murder spree once and for all.