The movie begins with the narrator recalling an awful event that took place
35 years ago in a town called North Vernon.
Back in October 1987, a girl named Tiffany, was brutally murdered in her garage on the
night of her 16th birthday party.
Merely 2 days later, another girl, Marissa, was found dead at her family cabin in the woods.
Then, on Halloween night, a third victim named Heather, was also brutally killed in the parking
lot of an amusement park.
Turns out all three victims were sixteen years old, and had been stabbed sixteen times,
so the killer is named “Sweet 16 Killer”.
It is then revealed that the killer never murdered anyone after that, and their identity
was also never found out.
Ever since then, the killer's mask has become very famous, and every Halloween,
people dress up as the Sweet 16 Killer.
In the present day, we're introduced to a local man named Chris Dubasage,
who hosts a podcast about these instances.
Every Halloween, he conducts a tour of the murder sites for visitors.
It almost appears that he has nothing else going on in his life besides this true crime event.
In the following scene, we see a teenage girl named Jamie,
who is getting ready to head out for a Halloween concert.
Moments later, her mother, Pam, shows up and forbids her from going outside.
It turns out that Pam was close friends with the three girls who were murdered in 1987.
So, even after 35 years, she is still scared of the killer.
However, Jamie insists on going, claiming that she is good at self-defense.
Her father, Blake, observes the two arguing and offers to drive Jamie to the concert,
just as he notices someone messaging his wife, and Pam claims it's just a mom group chat.
Later on, Blake brings Jamie to her best friend, Amelia's house.
There, Amelia’s mother, Lauren, is also present, and it's revealed that she is an
old classmate of both Pam and Blake.
Lauren complains that her boss does not allow her to rest even on Halloween,
and leaves for work.
After this, Blake accompanies the girls to the concert.
Meanwhile, Pam is alone at home, when a masked man appears on her doorstep.
She believes it is just another dressed-up guy, and offers him candy.
However, as soon as she notices a knife in his hand, she immediately closes the door.
She quickly calls the cops, but the man bursts through the door and attacks her.
Surprisingly, she also puts up a good fight, as she has been preparing for this day since
she was 16 years old.
After a while, Pam pulls out a knife, and threatens the killer
that the cops can arrive anytime soon.
This makes him somewhat scared, so he decides to leave.
But just as Pam exhales a sigh of relief, he suddenly appears from behind and strikes her.
He then continues to stab her exactly 16 times, until she passes away.
After her mother’s death, Jamie is deeply shocked and devastated, while Chris,
the true crime enthusiast, is broadcasting live outside the house.
The following day at school, the police arrive and ask her where her father was when the
murder took place.
They speculate that Blake may have been involved, because Pam and Chris often texted one another,
and maybe they were having an affair.
However, Jamie quickly shuts down the suspicion, claiming that her father could never do such a thing,
and blaming the police for failing to catch the killer 35 years ago.
After school, Jamie heads to a science fair, where Amelia is busy with a makeshift time-traveling machine.
Amelia shares that her mother, Lauren, had previously worked on a time-traveling project,
but never finished it.
Now, with her mother's old notes, she is determined to build a functional time-traveling machine.
She wants to test if they can travel back in time to prevent the killer's actions,
and Jamie adds that her mom would be still alive if they were able to prevent him.
To do so, Amelia has set the date to October 27 1987, when the first murder took place.
Later in the evening, Jamie watches Chris’s killing podcast, but her dad tells her to
stop listening to him since the man always turns tragic murders into entertainment,
especially the one from 35 years ago.
The next day, Jamie meets Chris at a cafe.
She directly asks him if he and her mom were having an affair, but he denies it.
He clarifies that Pam was deeply invested in solving the serial killer case,
therefore, they used to text each other.
He also reveals that Pam had received a note with a death threat from the killer after
the last murder 35 years ago, stating she would be the next victim.
Later that day, an upset Jamie returns to the science fair to meet Amelia.
But then all of a sudden, the killer shows up, and starts attacking Jamie,
but the girl manages to run and hide.
In an attempt to save herself, she accidentally enters the time machine, a.k.a the photobooth,
and she tries to turn it on.
Unfortunately, the killer follows her, and when he tries to stab her, he accidentally
plunges the knife into the photo booth stall's control panel.
This generates extra electricity, which instantly activates the time machine.
Luckily, before the killer can get her, the machine works, and Jamie is transported to 1987,
exactly to the day of the first murder.
Initially, she is a bit freaked out to realizes that she is in fact in 35 years ago,
but when she realizes that she can save her mother, she becomes determined to stop the killing spree.
Following this, Jamie places an "out of order" sign on the time machine booth,
and approaches a random woman nearby.
She inquires about her mother, Pam, and the woman informs her that she is at school.
Without wasting time, Jamie heads to the town's school where her mother was a student.
There, she introduces herself as an exchange student from Canada, and immediately gets
admission by the lazy clerk.
Later, in gym class, Jamie is shocked to see the three girls who were murdered,
Tiffany, Marisa, and Heather.
When the training starts, she struggles to look for her teenage mom, and to her surprise,
her patient mother that we saw at the beginning of the film is very different from what she imagined.
After the class, she discovers that her mother acts like she is superior to everyone else,
and is a complete bully.
Jamie also learns about Tiffany's birthday party that evening, where she is about to die.
She tries to convince the girls to cancel the party, claiming it's very lame,
but they don’t listen to her.
Frustrated, she goes to the local police for help, warning them that a murder is going
to happen at Tiffany's house.
However, the cops start laughing and claim that there has never been a murder in their town.
With no other option, Jamie starts searching around the school, and asks the lazy clerk
about Amelia's mother, Lauren.
She then enters Amelia’s mother class, and finds Lauren there, even though she’s a
bit confused by Jamie.
After she finds her, Jamie tells her about the time machine that her daughter will invent
in the future, and explains the entire situation.
She also informs Lauren about the upcoming murders and the details from her notebook.
Surprisingly, Lauren believes her because no one else knows about the secret notebook,
and her plan to build a time machine.
Jamie adds that she wants to stop the killer from killing people, including her mother,
and that the only way for her to return to the future is with Wi-Fi.
Since it doesn't exist in 1987, she needs Lauren's expertise to find an alternative solution.
The scene then shifts to 2023, where we see Blake frantically searching for Jamie and
talking to the police.
Just then, Amelia walks up to them, and reveals that Jamie used her time machine
to travel back in the past.
Naturally, no one believes her, but Chris is intrigued by what she just said.
Meanwhile, in 1987, Lauren and Jamie attend Tiffany's evening birthday party to prevent the murder.
At the party, Jamie meets a teenager named Doug, who always got bullied, and would later
end up as her principal.
There, Jamie notices an attractive guy, and is immediately smitten.
However, she is shocked when she discovers
that the guy is none other than her father, Blake.
She also learns that Tiffany and Blake are in a relationship, and her parents,
Blake and Pam haven't started dating yet.
After that, she tries her best to keep the three girls safe, but they don't seem to want
her help, and just keep making fun of her.
Moments later, Tiffany takes a boy upstairs for some fun.
However, she quickly goes to the bathroom for a while, but when she returns,
the guy is gone.
She hears sounds coming from the closet, and decides to wait for the boy to come out.
All of a sudden, the killer comes from the closet and walks towards her,
before brutally stabbing her.
During the loud music party, Jamie frantically continues to look for Tiffany to keep her safe.
Unfortunately, she finds her a little late, as her dead body is soon discovered in her
parents' bedroom.
Although terrified, Jamie realizes that she can change things in the past.
Tiffany was found dead in the garage in the regular timeline, but now,
in this altered timeline, she is found in her parent’s bedroom.
Afterwards, Jamie and Lauren walk Pam home, because they don't want her to be alone with
a killer on the loose.
When Pam asks her how she knew about the murder, Jamie claims she's psychic,
and that's how she predicts who will be killed.
The three then decide to stay together for the night, and Jamie asks Pam if she knows
anyone who might hate them enough to kill them.
In response, Pam admits that many people could be the killer, because she and her friends
have bullied a lot of people.
Back in 2023, we find Amelia working on her time machine, determined to figure it out
and bring Jamie back.
Suddenly, Chris arrives, and shows his interest in checking out the time machine.
As they talk, Chris suddenly realizes that he has been remembering the information incorrectly.
The brutal murder of Tiffany had actually taken place in the bedroom,
not the garage as he initially thought.
It turns out that Jamie's actions in the past are causing a ripple effect in the time continuum.
Finally, Amelia and Chris look at a picture from 1987, and notice that Jamie has indeed
traveled back in time.
Back in the past, Jamie tries to prevent the group from going to the cabin,
where the next murder will take place.
She convinces Marisa that they should go to the city instead.
Hearing this, Marisa says they can go to her parent’s condo instead, and Jamie agrees.
The teacher then assigns Jamie to partner up with the odd, frequently bullied boy named
Damon, and she notices the strange drawings in Damon's notebook.
Afterwards, Marisa and the girls pick Jamie up, and are on their way to Marisa’s parents’ condo.
But when they arrive there, Jamie is horrified to see that it’s not a condo, but the very
same creepy-ass cabin in the woods where Marisa died.
Jamie tries desperately to persuade them to leave, but the other guys show up, and they
decide to party at the cabin.
Worried, Jamie tries to lay down some safety rules inside the cabin, but no one pays any
attention to her.
Later that evening, one of the guys calls Jamie a party pooper,
and locks her out of the cabin.
Desperate to get in, she searches for a way when she stumbles upon a window.
Peering inside, she witnesses Heather, one of the three victims, lying on the bed, unconscious.
In the meantime, Marisa and the other guy decide to spend some time outside.
It is then revealed that the killer is already waiting inside the cabin;
he emerges out of the closet, and slowly approaches Heather while turning on the radio loudly.
Heather wakes up just in time to escape the room, but the killer catches up,
throws her down the stairs, and moves to murder her.
This changes the order of deaths, and instead of Marissa, it’s Heather who dies.
The other friends band together to stop the killer, but they are also beaten by him,
despite their survival.
After the killer runs out of the house and leaves them alone,
the teens head to the police station to report the murder.
Jamie tries to help by giving them the killer's blood for DNA testing, but the cops don't
even know what DNA is.
At school, Jamie talks to Damon, and it is revealed that he’s just a normal nerdy boy
who has nothing to do with the killer, and then Doug shows up
and tells them to get inside the class.
After the incident, the entire gang and Jamie begin planning how to catch the killer.
They decide to use Marisa as bait, since Jamie is aware
that she's the final person to be murdered in 1987.
However, this time, they will be ready to capture him the moment he appears.
On the other hand, Lauren needs enough force to power the time machine.
She will work on turning a popular attraction into a makeshift time machine for Jamie to travel forward,
while in the present day, Chris helps Amelia fix their machine.
The movie then cuts to Halloween night, where Marisa strolls through the amusement park,
while the killer discreetly follows her.
She enters a haunted house building, where the rest of the group is anxiously waiting
for the killer to appear.
Soon enough, the killer arrives, leading to a confrontation with Jamie and the others.
They all huddle in one room, where the gang has a hard time fighting the killer,
before Jamie finally manages to stab him.
Just when the killer is about to stab Jamie, one of her friends sneaks up from behind,
and fatally stabs him.
They then unmask the killer, who turns out to be their own classmate, Doug.
It is then revealed that a year ago, a student named Trish tragically died in a car accident.
Shockingly, the bullies, including Tiffany, Marisa, and Heather,
were responsible for her death.
They had nicknamed her "Fat Trish" due to her physique.
One evening, the three girls invited Trish for a sleepover at Tiffany's house, and planned
to get her drunk to make her talk about her rumored affair with their coach.
They continued to push her to drink more, even when she began to cry and was clearly
upset by the whole situation.
Eventually, Trish left the house in a drunken state and ended up getting into a car accident.
However, unknown to the girls, Trish was in a relationship with Doug, who was left devastated.
So, in order to avenge his love, he put on a mask and started killing the bullies.
Hearing all this, Jamie hates her mother, Pam, for being so mean, but Pam explains that
she wasn't at the sleepover because she fought with Tiffany.
She claims she would never do something so terrible, and Marisa verifies
that Pam was not involved in this.
So, Jamie is now confused about why Doug the killer would leave Pam a threatening note,
and eventually kill her in 2023.
But before they can process this, the second killer appears behind Marisa and murders her.
This guy follows Jamie, and kills Chris's father on the way, who was on live TV.
They eventually get into the time machine, which is now finally working, and Lauren explains
to Jamie that once the ride starts, she must stay within it.
And if she doesn't, she will be hurled against the wall and explode.
Unfortunately, as the two are conversing, the killer arrives and attacks them.
Jamie urges Lauren to leave immediately, which she does, and then Jamie turns on the machine.
All of a sudden, Pam arrives at the scene to help Jamie, and that's when the brawl between
them kicks off.
When the vehicle starts spinning fast, Jamie, Pam and the killer are pinned up against the
wall of the time machine.
But, the killer is familiar with the laws of physics, and manages to
walk over to Pam and is about to attack her.
He stabs her once, and Jamie pushes her mother out of the ride, when the killer's mask is
removed by the force, exposing Chris from 2023 as the perpetrator.
Here Chris admits that he had no idea that Doug was the original killer, and Chris murdered
Pam only to keep the Sweet 16 Killer story alive after 35 years.
He also gave Jamie a fake warning note in order to generate more content for his podcast
and make him more famous.
He was tired of being compared to his father, who was far more successful and prominent,
so he murdered him as well.
Chris then moves to attack Jamie, and the two engage in an intense fight.
Fortunately, our girl gains the upper hand, and shoots him with a nail gun.
Jamie kicks him into the spinning machine, killing him.
After this, she returns to the present.
Jamie arrives on the day before Halloween in 2023, and to her relief, Pam is still alive
and married to Blake.
Meanwhile, Lauren remembers everything that happened in the past, and informs Jamie that
this timeline differs from the one she came from.
Therefore, Lauren has created a book in which she has recorded everything that has changed.
Suddenly, a man greets Jamie and walks into the house.
Lauren then explains that in this timeline, Jamie's parents got together earlier than
they had originally, and now she has a 34-year-old brother named Jamie, and her name is now Collete.
To add, Lauren is a bioengineer and Amelia is still Jamie’s best friend.
Lastly, Chris in this timeline has been living in a monastery in India, traumatized by witnessing
his father's death on live TV, without realizing it was himself from a different timeline.