Rebecca Cantor, 16, writes a diary at a prestigious boarding school for girls, where she records her most private thoughts. To help her daughter forget about her father's passing, Rebecca's mother had her transferred to the school. Rebecca's father, a poet, committed suicide by cutting his wrist two years prior. Rebecca quickly recovers with the support of Lucy Blake, her flatmate, and best friend.
In the second year, the school welcomes Ernessa Bloch, a strange girl with black hair. Ernessa and Lucy quickly grow close. Mr. Davies, a brand-new English instructor, shows up at the school. Mr. Davies introduces himself as Rebecca's poet father's admirer. Rebecca is uncomfortable being around Ernessa. Lucy starts to drift away from Rebecca. Strange events begin to occur. Ernessa approaches Rebecca about her sinister secrets, but Rebecca dismisses her. Rebecca is jealous of Lucy and Ernessa. Rebecca finds Ernessa to be a mystery.
The next day in class, the English professor, Mr. Davies, hands out a book called “Carmella” to read. Soon after Rebecca starts reading, she discovers something about Ernessa that she never eats. Dora, Charley, and Rebecca find that she frequently hangs out in the basement, where students are not allowed to be.
Ernessa confronts Rebecca in the library and elaborates on the thrill of death while giving her a razor. As a result, she wonders how death can bring peace and how it feels when one slits their wrist. Ernessa gets into Rebecca’s thoughts and shares her history, including that Ernessa’s father killed himself and how hard it was for her to pass through. Soon after, Ernessa mysteriously turns to Charley to break the window glass. And Charley’s action led her to get expelled from the school without getting a second chance.
Rebecca, with the help of her friend Dora, walks on the roof, which leads to Ernessa’s room. She looks into it and notices an old room filled with moths. Rebecca and Dora intend to examine her room again the next night to confirm what she has seen. They both notice a supernatural power that Ernessa appears to be able to pass through closed windows. Ernessa is considered a vampire by Rebecca. Out of fear, Dora convinces her that what they saw was noise. The same night, Dora killed herself by falling out the window.
Returning from home after the Christmas holidays, Rebecca convinces herself to believe that Erneesa's actions were just her imagination, and she also apologizes to Lucy for being jealous of Ernessa and her friendship. Rebecca is a subject of particular importance to Mr. Davies. The two discuss Romantic literature and poetry in conversation.
Sofia, Kiki, and Rebecca discover a teacher is dead in the woods. As a result, school tension starts to increase. Rebecca's close friends were gradually expelled by Ernessa, forcing her to investigate the situation. Rebecca finds Lucy skipping all her meals. One night, Rebecca is shocked to discover Ernessa having sex with Lucy in the room. After that, Lucy was taken to the hospital, but only Rebecca knew Ernessa was to blame for her condition. Rebecca quickly discovers that vampires are not always bloodsuckers but are also capable of draining their victims' vitality. When Rebecca tries to explain this to Mr. Davies, he doesn’t believe her. Out of concern, Rebecca warns Ernessa to stay away from Lucy. The next day, Lucy returns from the hospital, doing better. In need of assistance, Rebecca turns to Mr. Davies. As they speak, they kiss, but Rebecca breaks it off.
Lucy refuses to listen when Rebecca tries to persuade her that Ernessa is the cause of all their issues and swears at her. She explains to Rebecca that she has changed from the "old Lucy" and that Rebecca's inability to recognize this has harmed their friendship. Despite Lucy's brief remission, she eventually passes away after Ernessa completely saps all of her vitality, and Lucy ends up dead.
Another time, Ernessa sings a terrible children's song about "The Juniper Tree" before slitting her wrist and drenching Rebecca with blood. Ernessa and the blood vanished after that. To end this, Rebecca steals the basement key from the school cupboard.
After stealing the basement's keys, Rebecca discovers an old suitcase with Ernessa's name scribbled on it. Rebecca discovers from an old diary that Ernessa's father also committed suicide years ago and that Ernessa followed suicide because she felt it had handled the pain. Rebecca finds out quickly that Ernessa had wished for her to commit suicide.
Ernessa is seen napping in the box when Rebecca returns to the basement. Before Ernessa awakens, Rebecca ignites the coffin and pours kerosene all around it. When Rebecca goes outside, she sees her classmates and a fire vehicle parked. She glimpses Ernessa's ghost through a door, who slowly circles and moves into the sun before dissipating.
Rebecca is sure Ernessa won't have left any traces, knowing the police are looking into her. She takes a razor blade out of her journal and throws it out the window as she drives to the police station while maintaining a blank expression.
