Steffani is an extremely mean-spirited teenage girl who lives her life treating others horribly. She is also the meanest girl at her school.
One night, while she and her best Friend Taylor Turner are playing incredibly loud music in her apartment room, Steffani gets a visit from her angry next door neighbor, Mrs. Biazevich a sick old woman who seems to dislike mean girls. Mrs. Biazevich tries to teach Steffani a lesson by brutally pulling her hair, but leaves when Taylor shuts the door preventing anymore violence from happening.
Steffani decides to get even with Mrs. Biazevich by crank calling her. However, the old lady knows it's only her and makes it clear that she'll make both girls pay. The next morning, Steffani and Taylor discover that Mrs. Biazevich has apparently died from her illness. But Steffani is too busy looking at a young paramedic to care. Later at school, Steffani starts receiving mysterious and freaky text messages telling her to be a nice girl and assumes they were from the young paramedic. When she answers a call, she and Taylor hear strange noises. They then find out that the calls are coming from Mrs. Biazevich's phone after sneaking in her room that night.
The next day, She and Taylor are very scared and decide to ask Adriana, a goth girl who Steffani usually picks on, for help. Steffani promises Adriana that she won't make fun of her anymore if she helps. Adriana tells the girls that her uncle Constantine might be able to help them and gives Steffani his address.
Upon arriving at Constantine's place, Steffani and Taylor are told that they're in danger due to the possibility that Mrs. Biazevich may be a ghost who's haunting them. Constantine gives them a vase full of leaves that will keep them safe from Mrs. Biazevich's ghost. That night, Steffani scatters the leaves around her bed to keep her and Taylor safe. However, it turns out that the leaves seemed to have attracted Mrs. Biazevich's spirit and she attacks the girls.
Mrs. Biazevich lets Taylor go because she can see good in her heart despite shyness, but sees that Steffani is still very mean, especially after she takes back her apology, so she traps her in a video in her cell phone and sends it to her granddaughter, who's revealed to be Adriana. It turns out that Adriana, Mrs. Biazevich, and Constantine planned this whole ordeal in order to teach Steffanni a lesson. Adriana sneers at Steffani trapped in the video and says "Steffani Howard Will never Mess with me, Again" and them deletes it, effectively killing Steffani by erasing her from existence.