In the school backyard, I-Na walks among a bunch of desks on fire and is suddenly attacked
by a creepy girl that reminds her of her friend Ji-Won.
More ghosts appear in the area and I-Na finds herself surrounded, but when she thinks she's
about to die, she wakes up in her bed at the school dorms.
Things are tense on the whole campus because this is the last week of exams and everyone
is under pressure.
I-Na has less patience than usual for the trust fund kids, who get extra privileges
in the school just for having money, but she gets to relax at least a little bit when she
hangs out when her best friend Yoon and school prankster Hyun, who has a crush on her.
The teachers are tense as well: Choi hasn't smiled in forever, and Hwang keeps scolding
Lee for being too harsh on the students.
During an exam, Beom keeps hearing whispers around the classroom and suddenly yells when
he sees a creepy face appearing on his test, causing him to fall off his chair.
Hwang comes to check on him, obviously finding nothing on the paper, but he does notice the
worrisome scars on Beom's wrist, although he says he didn't do it and that "it was her".
Moments later, a funny song suddenly starts playing from the speakers.
The teachers don't allow the test to be interrupted, but they discover this is one of Hyun's pranks,
and as punishment he's asked to come to school over the weekend to help the teachers with
some chores.
In the afternoon, the teachers have a meeting with the principal, who informs them students
from their sister school will be visiting them in ten days, so he wants them to make
a temporary class with their top students and start training them this weekend.
The following day, the test results are posted and I-Na and Yoon discover they'll be part
of the special weekend class, which is a big deal considering the top three as usual are
taken by the rich kids.
When Beom comes over to find out his own grades, he begins feeling dizzy and notices the creepy
girl from I-Na's dreams in the middle of the crowd.
After a few seconds, the creepy girl's face fusions with I-Na's, causing Beom to attack
her.
Other students immediately pull him away and an ambulance is called so he can be taken
to a mental hospital; as the nurses drag him away Beom yells at everyone to tell them they'll
die soon.
In the evening, guard Kim is doing his usual rounds when he suddenly hears a noise coming
from under the building floor: it's Beom, who has escaped the mental hospital and has
shown up at school with his hands covered in blood.
He's obviously scared and keeps staring at something, but Kim doesn't know what.
When the guard moves his flashlight for a better look, Beom gets a full view of a decaying
body and runs away as he screams.
The next day is Saturday, and the special class begins with the teachers asking student
Song to collect everyone's phones since they won't be allowed during this special day;
Hyun has also come over to work on his punishment.
When the class begins, Hwang notices a girl called Min is missing but assumes she's just
late and begins the lesson anyway.
Meanwhile Song is on his way to take the phones to the staff room when he hears a mysterious
voice calling his name.
He follows the voice to the bathroom where he's shocked to be suddenly attacked by an
unseen force that heavily wounds him before dragging him by the ankle under a stall door
and then into the ceiling.
Back in the classroom, Choi begins her lesson by playing an educational video that gets
abruptly interrupted by a terrifying sight: Min is trapped inside a glass container that
has an equation written on its side.
Not taking it seriously yet, Choi tries to turn it off to no avail and sends I-Na to
the staff room to check what's going on.
As she runs there, I-Na notices every screen in the hallway is showing the same thing,
and when she makes it to the staff room she discovers the internet has suddenly gone down.
At that moment, a creepy voice comes from the TVs announcing everyone will be taking
a new kind of test; if they don't answer the exercises then students will die one by one.
If anyone leaves the campus, they'll also die; and all questions must be answered to
get the full answer.
Teachers and students are equally confused, not knowing what to make of this.
To make matters worse, Kim comes in to confirm all phone lines have been cut off.
None of the students have their cell phones with them, and the teachers discover theirs
have been broken.
Kim is sent to keep an eye outside while I-Na and the teachers join the others in the classroom,
where everyone is screaming because the glass container Min is in has started to fill with
water.
Thinking this may be another prank, Lee attacks Hyun and Hwang has to pull them apart to stop
things from getting violent.
An argument begins among the teachers and everyone is so distracted by this that they
forget about the equation until it is too late: once the container is full of water,
Min drowns.
Seeing a real death convinces Lee that Hyun didn't do this, and he decides to go across
the street to see if he can find a phone.
Choi's reminders of the rule are ignored and the students decide to copy Lee, but as they
soon as they make it to the reception, they discover the rule had been true.
Lee comes back covered in blood and dies as soon as he crosses the door while the voice
reminds them they need to stay inside.
Meanwhile Beom has managed to get inside the building too and he wanders around, only stopping
to doodle on a notebook while he hears more noises.
Next, the creepy voice informs everyone their next exercise will have an audio clue and
hurts Song to make him scream into the microphone, it also makes him confess that his "sin" is
having a greedy mother.
Then the voice announces that the exercise can be found in the auditorium, where they
must put together a phrase related to greed using the words waiting for them on the floor.
After lots of brainstorming, I-Na and Hwang manage to think of a fitting sentence, but
they don't make it in time and Song's body is dropped in the middle of the auditorium,
his face covered by the candle wax the bad guy used to hurt him and the answer to the
riddle written on his chests by scars.
On top of it all, a student named Cha is now missing too, which gives I-Na an idea that
makes her run out of the auditorium.
Hyun follows her and finds her looking at the students' ranking, which confirms her
theory: they're being killed in the ranking order.
Min and Song had first and second positions, and the missing Cha is the third.
Yoon will be the fourth, and I-Na herself comes fifth.
In the auditorium, the rich kids think they would be safer if they hid in their dorms
so they make a plan to sneak there; Yoon overhears this and joins them.
They get permission to leave the auditorium from Hwang by pretending to need to go to
the toilet, and with a quick run they make it to the dorms, unaware that Beom can hear
them running through the hallways.
At that moment, I-Na and Hyun return to the auditorium and tell the teachers about their
discovery, meaning they need to find Yoon as soon as possible to protect her.
They end up diving the class into two groups, one guided by Choi, and the other by Hwang.
By the time night falls, Cha has been missing for six hours and there's been no progress
in the search.
Suddenly, something begins shaking the door of the dorm where the escaped group is hiding,
and the strange force creates a hole in the wood in order to grab a student's hair and
pull his head through, instantly killing him.
Meanwhile Hwang's group sees a light being turned on at a random classroom and goes to
check on it only to discover the next exercise: looking at the pictures on the board, they
need to decode a password to use on the computer on the desk.
While they get down to business, Choi's group sees lights flickering at a dorm window and
rushes there to find the students panicking in the hallway.
Everyone's yelling and pulling at each other, not knowing what's going on and not seeing
Beom hiding in the stairs because he can see the creepy ghost in the hallway with them.
After a few moments of utter chaos, the lights start working normally again and they discover
that the attacker made a mistake: they kidnapped a girl named Soo-Jin instead of Yoon, who
never left her bedroom in the first place.
In the classroom, Hwang's group finds out the answer is "on a warm spring day, remember
the auditorium".
They put the password in the computer, but it's already too late: Cha is dead, and the
body appears on camera together with the decaying face of a creepy woman; such horrifying sight
makes them scream and run out until they finally reunite with Choi's party.
Afterward, everyone goes to the cafeteria so they grab a bite.
While discussing the clue from the password, I-Na realizes this is related to her former
best friend Ji-Won.
She used to be the number one student in their class because she always studied really hard
in order to keep her scholarship since her family was poor to afford this school.
One day, when the ranking was posted after another important exam, everyone was shocked
to see Ji-Won got sixth place, which meant she would have to leave the dorm after the
next term.
A few days later, Ji-Won showed up dead in the auditorium under mysterious circumstances.
The students in the cafeteria begin to wonder if Ji-Won's ghost is doing this to them, causing
a clearly uncomfortable Hwang to drop the subject because ghosts don't exist.
After Kim comes to join them for dinner too, a melody starts playing somewhere in the building
announcing the next test.
Hwang decides he's had enough and goes looking for the murderer while Choi, I-Na, and Hyun
follow the song to the staff room, where a puzzle awaits them in the form of an anagram.
To solve it, they must put the pictures together to recreate a photograph of the school.
While Soo-Jin is also heavily wounded by the attacker before she's put inside a washing
machine, Hyun retrieves a matching picture from the reception to use as a guide during
the solving of the puzzle.
The plan works and the answer is revealed to be "I know who killed me, my anger will
endure, face the truth".
Meanwhile Hwang keeps hearing weird noises outside and seeing shadows in the corner of
his eyes.
Suddenly, a mysterious force attacks him and pushes him away, so Hwang runs to hide in
a closet and has to make a huge effort not to scream while a knife keeps stabbing the
door.
Luckily, the attacker goes away and Hwang finally gets to escape, bumping into Choi
and the teens on his way out.
Before they can share information, they begin hearing a beeping sound that they follow to
discover it comes from a washing machine announcing the end of its cycle.
Inside, Soo-Jin lies dead.
Choi is more sure that this is connected to Ji-Won now, but when she turns to I-Na for
her opinion, she finds her gone.
In the cafeteria, Yoon realizes everyone around her has abruptly fallen asleep, but instead
of worrying about what it may mean, she puts on the headphones of her music player to distract
herself.
This results in Yoon not hearing the decaying woman approaching her from behind to kidnap
her properly this time.
At the same time in the dorms, I-Na follows a mysterious whisper until she finds the creepy
girl from her dreams drawing on the doodles Beom made earlier.
The ghost tries to grab I-Na and makes her scream, but by the time the teachers and Hyun
find her, the creepy girl is gone.
Now they need to think of a serious plan, so Hwang asks Hyun how he pulled off the prank
last time.
Once the boy explains the kind of connections he made, the group goes to the PA room and
finds the wire that runs throughout the whole school for all the screens, thus they follow
it underground to see if it can take them to the creepy woman.
However the first person they find is Beom, who doesn't hesitate to attack I-Na as soon
as he sees her.
Hyun doesn't hesitate either and comes to I-Na's defense, but in the struggle, Beom
stabs him until he dies before turning to I-Na again.
This time it's Hwang who stops Beom, and in a fit of fury, he kills Beom with multiple
hits.
There's no time to grieve because the melody announcing a new riddle plays once more, this
time together with Yoon's screaming.
I-Na and the teachers follow the noise into a hallway with a locked door at the end, to
open it they must solve the exercise written on it.
Behind this door Yoon is hanging from her ankle while having to watch a dead Min float
and a candle slowly burn the rope that keeps her up.
Fortunately Choi recognizes the necessary math and quickly discovers the combination
number, allowing them to open the door and rush inside.
While Choi finds a bunch of paper sheets with all the tests on them, Hwang tries to find
a way to put Yoon down safely and I-Na hugs her to comfort her through her fear.
Sadly the creepy woman doesn't care that they solved the test, she still brings Yoon up
before letting her fall in one go, causing Yoon to die as soon as she touches the floor.
When Hwang sees there are steps he can take to the roof, he climbs them and finds the
creepy woman, who tries to stab him.
Luckily Hwang thinks quickly and fights back, managing to kill the woman after some struggle
and finally putting an end to all this.
Sometime later, a memorial for the lost students is held in the auditorium.
I-Na can't stand the overwhelming pain and goes to catch some air on the roof, where
she's shocked to find the paper sheets with the test plans.
There's also a bigger surprise: a photograph of Ji-Won with her family, revealing her parents
were the creepy woman and Kim the guard.
Speaking of Kim, he jumps on I-Na from behind and kidnaps her, showing he had been working
with his wife all along and he wants to see their plan through to the end.
After tying I-Na to a chair, Kim fills the auditorium with gas as he begins to talk through
the speakers.
He admits to being Ji-Won's father and that he's happy all the rich parents finally know
the pain he's been suffering for two years.
Then, he plays on the big stage screen a recording of Ji-Won being murdered that he had found
on her phone; he also asks the murderer to reveal himself.
While everyone in the auditorium panics about possibly being in the company of a murderer,
it is revealed that the person that killed Ji-Won had been Hwang.
It turns out Hwang was paid by the rich parents to give their kids better grades, and that
was the only reason Ji-Won had gotten sixth place.
She confronted Hwang to ask him to make things right or she would go to the principal, thus
Hwang reacted by killing her.
All this had been seen by Beom, who watched instead of helping, and that's why since then
he imagined Ji-Won followed him everywhere.
In the auditorium, Hwang finds himself cornered and goes crazy, grabbing the emergency ax
from the wall and threatening anyone that comes close.
Before he can hurt anybody though, Kim shows up and gets his revenge by taking the ax from
Hwang and killing him slowly with it, ignoring the teacher's apology.
Kim's flashbacks reveal he had killed the original guard so he could take the job, meaning
he's been planning this since Ji-Won's death two years ago.
Seeing as Kim is there, Choi runs out of the auditorium and rescues I-Na.
A few weeks later, I-Na checks on the student ranking and is pleased to see she's won first
place.
But as she turns around, her face and Ji-Won's become one, implying she may've been possessed
all along and that's why Beom kept attacking her.