At an exclusive boarding school for girls, 16-year-old Rebecca writes her most intimate thoughts in a diary. Two years earlier, Rebecca's father, a poet, took his own life by slitting his wrists. Her mother transferred Rebecca to the school, hoping to help her daughter escape the memory of her father's death. With the help of her best friend and roommate, Lucy, Rebecca soon recovers. The following year, a mysterious, dark-haired girl named Ernessa Bloch enrolls into the school. Lucy quickly becomes best friends with Ernessa and becomes distant from Rebecca. Ernessa's presence makes Rebecca feel uneasy. She tries to confront Lucy about Ernessa's dark secrets, but her pleas are dismissed as jealousy. Eerie things start to happen. First, Charley gets expelled because of Ernessa. Dora dies in a freak accident shortly after spying on Ernessa's room, and a teacher is found murdered in the woods. Tension starts to grow at the school. To Rebecca, Ernessa is an enigma. She seems like she can walk through closed windows, and she is often seen lingering around the basement. Rebecca thinks Ernessa is a vampire. Ernessa slowly gets rid of Rebecca's close friends, leaving Rebecca to find out what is happening by herself. A new English teacher, Mr Davies, arrives at the school. Mr Davies shows particular interest in Rebecca. The two share ideas on Romantic literature and poetry. Rebecca soon learns that vampires do not necessarily drink blood, but they can drain the lively spirit out of their victims. Mr Davies addresses himself as a fan of Rebecca's poet father. Rebecca turns to Mr Davies for help, and, during their conversation, the two kiss but Rebecca pulls away. Ernessa confronts Rebecca in the library and presents her with a sharp razor and elaborates on the pleasure of death. Another time, Ernessa sings a disturbing nursery rhyme about "The Juniper Tree" then slits her own wrists, causing blood to rain down on her and Rebecca. Afterwards, Ernessa and the blood disappear. Lucy is sent to the hospital, but only Rebecca knows that Lucy is sick because of Ernessa. Rebecca tries to convince Lucy that Ernessa is the root of all their problems, but Lucy refuses to listen and profanes at her. She tells Rebecca that she is not the "old Lucy" anymore, and that Rebecca's refusal to see this is what has really spoiled their friendship. Although Lucy recovers for a couple days, she soon dies after Ernessa completely drains the life out of her. Rebecca steals the keys to the basement, and after entering, sees an old suitcase with Ernessa's full name written on it. From an old diary, Rebecca learns that many years ago Ernessa's father also killed himself, and Ernessa, unable to cope with the grief, took her own life thereafter. Rebecca soon learns that Ernessa has wanted Rebecca to kill herself. Shortly after, Rebecca returns to the basement to discover Ernessa sleeping in the suitcase. Rebecca pours kerosene on Ernessa and around the coffin and lights it before Ernessa wakes up. Rebecca walks outside to see a fire truck present and her classmates standing around. Through a door she sees the ghost of Ernessa, who slowly turns around and walks into the sun before vanishing. Knowing the authorities are suspicious of her, Rebecca is certain that Ernessa will not have left any remains. During the ride to the police station she pulls a razor blade out of her diary and drops it out of the window, staring blankly into the distance.
The Moth Diaries received primarily negative reviews from critics. On Rotten Tomatoes, the film has a rating of 13%, based on 46 reviews. On Metacritic, it received a score of 38 out of 100, based on 13 critics, indicating "Generally unfavorable reviews". Neil Young of The Hollywood Reporter criticized the films lack of narrative suspense and overall inert storytelling, stating that "The Moth Diaries is about as scary and menacing as the harmless lepidoptera in the film’s title." On 4 May 2015, Scout Tafoya of RogerEbert.com included the film in his video series "The Unloved", where he highlights films which received mixed to negative reviews yet he believes to have artistic value. He stated that "unlike other young-adult adaptations, the ritual and hardship of being in high school is never edged out by the supernatural goings-on; Rebecca and her friends worry about boys and grades as much as vampires... Harron was attempting to communicate honestly with teens and pre-teen girls, to make a movie with characters and situations they might recognize." Tafoya further added, "Where the world refuses to stop turning just because crisis mounts for one girl, and thanks to a careless if not downright malicious marketing and distribution strategy, its audience never got a chance to watch it alongside Twilight sequels or Marvel movies... The film industry is only just learning it can't get away with ignoring women of all ages."
