The story begins with a memory of a girl trying to hurt herself at school while other girls watch. When Myung Ja-eun, one of the girls in the class, tries to help her, the troubled girl threatens to hurt Ja-eun with a knife. Ja-eun tries to stop her but gets a cut on her neck, and blood begins to fall to the floor. The scene then moves to today, as Sung Soo-ji starts at Baekyeon Girls’ High School and is late. She lies to the supervising teacher and tells her that she was informed to arrive 10 minutes later than the school time, with that line she manages to get in. As Soo-ji comes up the stairs, she bumps into a girl who steps on her shoes; the girl somehow knows Soo-ji is a new student and instructs her to go to the teacher's office. There she meets Mr. Lim, the class teacher, who tells her she'll be in Year 2 Class 5. Soo-ji notices that other teachers in the room also look at her weirdly. Mr. Lim brings her to class 5, but it is secluded and doesn’t stay in the same building as the other classes; in fact, it is the only class in that building. As a soldier’s daughter, Soo-ji is used to moving to new schools. Mr. Lim introduces Soo-ji to the class, and she says hi to her new classmates but notices they don't seem friendly. As someone who changes schools a lot, Soo-ji finds it easy to adapt to a new environment and tells herself to only avoid two types of people in the class: the losers and the bullies. She ends up sitting next to a girl who is lying her head on the table, who turns out to be Ja-eun, the girl we see in the beginning. Soo-ji notices that Ja-eun has a lot of injuries and cuts on her body. Soo-ji tries to make friends with Ja-eun, but she doesn't want to. However, some girls, including a beautiful girl named Ha-rin, approach and start a conversation with Soo-ji. They also introduce her to Ye-rim, a girl in the class who is training to become a music idol, and Seo Do-ah, the class leader with top results. Do-ah then tells everyone to go to the auditorium for Music Class. There, Soo-ji sees the teacher scold Ja-eun for being late. Then the classmates cheer for Ye-rim and ask her to perform. A classmate sitting next to Soo-ji tells her that Ye-rim will soon debut. She is very well liked in the class also good at school. At the end of the day, Soo-ji is leaving the classroom and says bye to everyone, and they joyfully say it back, but their facial expressions swiftly change as soon as Soo-ji leaves. The following day, Soo-ji thinks about where she fits in with her classmates and tries to blend in. She really doesn't like Ja-eun and decides to stay away from her. Time goes by, and Soo-ji feels more like she belongs. However, she starts to notice something strange: Class 5 is also the only class in the cafeteria and the girls from class 5 don’t have any interaction with students from other classes. One day, Soo-ji witnesses Ja-eun being tied up and tortured in the sports room by Da-yeon, Woo-yi, and Seol-ha. She doesn’t interrupt them because she's afraid she will become their next target. When they leave, Soo-ji comes in and tells Ja-eun to report them instead of suffering and gives Ja-eun some bandages. Soo-ji sees some wrist-cutting scars on Ja-eun's arm, showing that she has tried to harm herself multiple times. Then, Ja-eun asks Soo-ji what day it is. As soon as she realizes it's the last Thursday of the month, Ja-eun rushes Soo-ji back to the classroom. Soo-ji enters the class and sees everyone staring at her. Do-ah comes up to the board and draws a triangle, then sends Soo-ji a link to download an app called Pyramid Game. She explains that every last Thursday of the month, everyone in the class will play this ranking game. They can also choose not to play, like Ja-eun. Everyone who plays will vote for 5 people from the class. Based on the number of votes, each student can be placed in ranks between A and D. People of rank A will enjoy certain privileges over those with a lower rank, and one of these privileges is that someone who is ranked lower can't fight against people with rank A. Anyone who doesn't receive any votes will be ranked F and become the outcast of the class. Soo-ji is hesitant to join this stupid game but is still sure she will receive at least a few votes from some people she usually hangs out with, so she votes for them. However, she soon realizes that no one votes for her, making Soo-ji an outcast with Rank F, and she will be the next victim for the bullies in the class, which includes Da-yeon, who gets Rank A. At home, she asks her dad if they will move soon, but her dad says they will stay here for three months, so Soo-ji understands that she has to endure the bullying for three months before it will be over. At school the next day, Soo-ji starts getting taunted by Da-yeon's group. When Soo-ji tries to act tough, Da-yeon has her friend Seol-ha attack Soo-ji and slam her to the ground multiple times in front of the whole class, but no one interferes or helps her. Not only that, when Soo-ji goes to the restroom to clean herself up, she notices how the bullies have left no bruises on her, so she can't report them. Da-yeon keeps taunting her in the class with a dog whistle, but everyone in the class just ignores it as it’s normal for Rank F to get bullied. Later, as Soo-ji is eating in the canteen, Da-yeon comes in and forces her to take an addictive painkiller pill. Soo-ji dares her to play a guessing game on whether Soo-ji took the pill or not and wins, pissing off Da-yeon. After that, Soo-ji decides to use the CCTV footage on their floor to gather evidence of the bullying. However, to her surprise, the CCTV cameras have been taken down. She turns on voice recording on her phone intending to get the evidence of the bullying herself. She bumps into Ha-rin on the way back to the class, and Ha-rin warns her that Da-yeon is looking for Rank F people and tells her to stay strong. When she gets in the class, Woo-yi realizes she’s recording, snatches her phone to turn it off. using Soo-ji's phone, Woo-yi learns that she's on her period and takes the sanitary pads out from Soo-ji’s locker and cuts them up in front of her. She also makes sure the other students don’t lend pads to Soo-ji. Day after day, they torture Soo-ji with a new method and make the teacher think that they actually befriend her. However, when Yerim and Ha-rin pass by, they stop and go away, but Ha-rin comes to check on Soo-ji again. Da-yeon has an idea that from now on, anyone below Rank A can also bully Rank F. And all of them enjoy doing so as soon as they’re allowed to, Soo-ji understands that most of the classmates she has are mean and they just use the game to justify mistreating others. Soo-ji finds it impossible to survive in this hostile environment and decides to find a solution to win the Pyramid Game. Meanwhile, Ja-eun contemplates ending her life by jumping into an empty swimming pool but gets scared by her own idea and steps back. Soo-ji looks into the Pyramid Game ranking and learns that Ye-rim, despite being the most popular girl in class, was a Rank B student, while Da-yeon and Do-ah were Rank A. She also notices how Ja-eun and she were the only two students with 0 votes. When ja eun comes in, soo ji then loudly says thanks to Ja-eun before the others, making the whole class turn their heads. Soo-ji tells Ja-eun that she wants to be besties with Ja-eun and help each other get out of Rank F. However, Ja-eun pulls her out of the class; she is surprised to learn that Soo-ji is also Rank F and gets bullied. Soo-ji tries to convince Ja-eun to team up with her to bring down the two girls that got 1 vote and replace them in Rank D. However, Ja-eun refuses because she doesn't want anyone else to get hurt. Soo-ji suspects others, not just the bullies, control the Pyramid Game from behind the scenes. Meanwhile, the bullies seem to be reporting about Soo-ji to someone, a student wearing a pair of Gucci sneakers. Soo ji runs into Eun-byeol, the class loser who asks Soo-ji to meet her in the Music room in private. Eun-byeol says she used to be friends with Ja-eun but swapped votes with someone else. This left Ja-eun with no votes and made her Rank F. Eun-byeol wants to help Ja-eun get out of Rank F. Soo-ji is in English class, thinking about her conversation with Eun-byeol earlier. Eun-byeol mentioned that Ye-rim comes from a rich and influential background but never talks about it. Soo-ji tries to recall what Eun-byeol said about the others when Da-yeon approaches her with a dog whistle. She has her two friends hold Soo-ji down and pour a box of worms into her mouth. Soo-ji pushes them away and spits the worms out. Woo-yi tries to pick up the worms from the ground, she wants to put them in soo ji’s clothes. She hums a song, which reminds Soo-ji of something, and she tries to remember it. At the same time, a student from Class 5 is smoking in the office of school board member Choi Yi-hwa. Her face isn’t shown, but she is wearing a pair of Gucci sneakers. Ja-eun shows up there, and the student tries to poke the burning cigarette into her eye. Ja-eun asks the girl to stop and agrees to do anything she wants. A flashback from one month ago shows the school principal talking to the mysterious girl with the Gucci sneakers about Soo-ji, the new transfer student who’s going to join the school. The girl asks the principal to place Soo-ji in Class 5. Back in the present day, the mysterious girl enters the class with Soo-ji on the ground. The girl is none other than Baek Ha-rin. Soo-ji recalls her humming the song, just like Woo-yi does. She also got the most votes and planned to have Soo-ji placed in Class 5 as her new bullying victim.