The story begins with the pledge; Eon-Joo, So-Hee, Yoo-jin and Eun-young pledge that they will die together that night and that if for some reason they didn't then they would still keep their pledge to die. Eun-Joo jumps from the roof of her Catholic school, committing suicide and falling to her death. Her sister Jeong-Eon, waiting for her outside, sees her jump. The next day in the school office; Jeong-Eon, So-Hee, Yoo-jin, and Eun-Young all swear that they all left for a while, and that Eon-Joo wandered off to go to the restroom then jumped off the roof. So-Hee admits that she was on the roof when Eon-Joo jumped.
The three girls become very paranoid about the death of Eon-Joo, and secrets about why each girl had a reason to jump start to surface. So-Hee becomes very depressed about the death, unlike the other two. During the funeral of Eon-Joo, So-Hee leaves crying. Jeong-Eon catches So-Hee before she leaves and confronts her about the death of Eon-Joo but So-Hee repeats that she doesn't know what happened and leaves.
Meanwhile, rumors start to spread around the school. Many girls accuse So-Hee of killing Eon-Joo as their relationship had become strained from the past year. Another girl claims that Eon-Joo killed herself because she was pregnant. The three girls meet in a corner of the school, each accusing the others of spreading the rumours, but they all deny it. Eun-young screams, claiming that she saw someone watching them. That evening Eun-young is lying awake in bed terrified, and her father comes in and beats her for having low grades. The next day she attends school with an eye patch over her left eye and begs Yoo-jin to let her sleep at her home.
So-Hee has flashbacks of the time when she was friends with Eon-Joo, before she became friends with Yoo-jin and Eun-young, and feels guilty. She goes to Eon-joo's classroom to pray but is confronted by Joo-yeon, who tells her that because she pushed Eon-joo away she was to blame for her death. So-Hee leaves and while Joo-Yeong keeps rambling about it to herself, Eon-Joo appears behind her and kills her. When So-Hee gets back, she sees nothing but the window open.
Later, Eun-young speaks to two girls in the bathroom, gossiping about Eon-Joo's death. Eun-young sees Eon-Joo standing in an open bathroom stall; the left side of her face bloody and cracked. Eun-young runs to Yoo-jin in class and screams that she has seen Eon-Joo in the bathroom. While Yoo-jin comforts her, Eun-young sees Eon-Joo again outside the window, screams, and then wets herself. After a small fight, Eun-young grabs her stuff, wanting to talk to a teacher. Yoo-jin locks Eun-young up, believing she would be safe there, and goes to computer class. Eun-young screams for Yoo-jin when she sees Eon-Joo outside the door. In her classroom, So-Hee hears a buzzing sound then notices a strand of hair in one of the lockers, she opens it and finds a dismembered body, possibly Joo-Yeon's. We see Yoo-jin in the computer room and computer screens go black. However, Yoo-jin's screen shows Eon-Joo standing over Eun-young, who is curled up. Yoo-jin falls from her chair and goes to rescue Eun-young. She finds her in the bathroom, washing the urine off of her leg. Yoo-jin asks who let her out and she tells her that Eon-Joo let her out, and that Eon-Joo promised she wouldn't kill any of them if they came clean. Later, Yoo-Jin and So-Hee see Eun-young on the roof. They beg her not to jump but she does, claiming that she is no longer afraid, and that she feels good because she won't have to go back to her abusive home. So-Hee holds Eun-young to comfort her. The faculty ask So-Hee what happened but she can't speak.
Later on So-Hee begs Yoo-jin to let her confess but Yoo-jin storms out of the classroom. So-Hee stays behind, and Eon-Joo is seen behind her. She turns So-Hee's head around to look at her and So-Hee screams. Yoo-jin runs back to check on So-Hee, then asks her to go to the church to try apologize. Yoo-jin attempts to take the church key that used to belong to her before Eon-Joo beat her in grades but doesn't find it. Jeong-Eon asks Yoo-jin if she is "looking for this" and holds up the key. Jeong-Eon tells Yoo-jin that she knows she was on the roof with Eon-Joo. Yoo-jin asks how she knows that, and Jeong-Eon responds "Eon-Joo told me, she told me everything!" Yoo-jin hits Jeong-Eon with a statue, knocking her out, and leaves.
Inside the church, So-Hee enters the confession box. Yoo-jin pretends to enter hers but instead goes into So-Hee's box. She smiles and says, "Do you want to know why we asked you to kill yourself with us?". A flashback shows Soo-Hee with her boyfriend, Ki-Ho. Yoo-jin used to date Ki-Ho and immediately made So-Hee and Eun-young stop being friends with Eon-Joo, despite them being such great friends. In the present, Yoo-Jin paces in the church, telling So-Hee and Eun-young that she would kill herself if she didn't make top grades, not because she was terrified of losing the key, but losing to somebody in general. Eun-young sits up and says "Suicide isn't funny. I tried to kill myself last year because of my dad." So-Hee then confesses her pregnancy. Later Eun-young and Yoo-jin sit together in a bedroom. Yoo-jin suggests that because So-Hee is better at school work than Yoo-jin, she doesn't want to lose to So-Hee again. Yoo-jin and Eun-young decide to pretend to jump.
Yoo-jin drags So-Hee to the front of the church, tying a microphone around So-Hee's neck and stringing her high up. A girl storms in screaming Yoo-jin's name. Yoo-jin asks "Which one are you? Jeong-Eon or Eon-Joo?" It turns out to be Eon-Joo, who chases Yoo-jin to the back of the church; Eon-Joo catches her, and strangles Yoo-jin. Eon-Joo steps forward to So-Hee who screams. In a flashback, Eon-Joo had told So-Hee that she wanted to die together with So-Hee one day, but regardless of this, she was no longer allowed to talk to her. However, on the day of Eon-Joo’s suicide, So-Hee called Eon-Joo to the library and told her that she was sorry for everything and confessed her pregnancy. So-Hee swore that she was going to kill herself and take the baby with her, and Eon-Joo agreed to die along with So-Hee no matter what. So-Hee confesses to Eon-joo that she was going to commit suicide and the four girls meet on the roof, holding hands. Before the jump, Eun-young and Yoo-jin step back, So-Hee realizes and tries to stop her but Eon-Joo jumps. So-Hee tries to keep hold of Eon-joo's hand, but Eon-Joo falls to her death.
The next scene is of So-Hee crying with Eon-Joo on the roof. Eon-Joo smiles and tells So-Hee that she missed So-Hee her and that she wanted to return, but knew she would scare So-Hee, then So-Hee hugs Eon-Joo and said she was sorry. Eon-Joo tells So-Hee she can no longer return while So-Hee begs Eon-Joo to take her with her. However, Jeong-Eon grabs So-Hee's hand, and Eon-Joo tells So-Hee that she must stay to take care of Jeong-Eon and that So-Hee is now Jeong-Eon's older sister.
In the final scene So-Hee has decided to keep baby and Ki-Ho has moved on with a short-haired girl. The two step into an elevator, and as the door is closing, the girl's face turn into Eon-Joo's.Son Eun-seo as So-Hee
Jang Kyeong-ah as Eon-joo
Oh Yeon-seo as Yoo-jin
Song Min-jeong as Eun-yeong
Yoo Shin-ae as Jeong-eon
Park Ga-hee as Joo-yeon