Four years after the passing of her father, David, Emma Grossman (Mckenna Grace) has transitioned into her teenage years while living with her uncle, Robert (Anthony Edridge), aunt Angela (Sophie Vavasseur), and their infant son Cade. Despite her seemingly ordinary life filled with school, friends, and the occasional boy trouble, Emma conceals a dark secret; she is a cunning psychopathic murderer who eludes capture while carrying out cruel acts of violence.
As Emma prepares to audition for the school dance team, she hatches sinister plans against her family and peers. For instance, she attempts to drown Cade by exploiting an open back door leading to the pool, only to have Angela rescue him at the last moment, directing blame towards Robert for his negligence. When Robert grows suspicious of Emma's involvement, she escalates her malicious behavior, nearly killing him by sabotaging his car.
Things take a turn as a new girl at school, Kat infiltrates Emma's group. Kat provocatively accuses Emma of past bullying and reveals her knowledge of Emma's sinister deeds. Meanwhile, Emma faces jealousy when her friend Steph is named captain of the coveted dance team position, leading her down a deadly path where she cruelly murders Steph's dog and causes a seizure in class, ultimately resulting in Steph's tragic death.
Angela, growing increasingly alarmed about Emma's erratic behavior, uncovers film evidence of Emma's involvement in the dog's disappearance and confronts her. Emma brazenly admits her actions but maintains her innocence about harming the dog. During a conversation with Brian, the property caretaker who has links to Emma’s past, Angela learns that David had feared Emma would commit more atrocities. This realization leads Angela to understand Emma's psychological disorder; Emma herself, after exploring the Hare Psychopathy Checklist online, revels in this newfound self-awareness.
Emma meticulously plans to eliminate Kat, enticing her over under false pretenses, only to poison her hot chocolate. As chaos unfolds, she maliciously orchestrates a fire, all while taunting Robert who fights to rescue Cade, only to find Emma has deceived him with a recording of Cade's cries. In a tragic twist, both Angela and Robert perish in the flames as they attempt a rescue.
In the aftermath, Emma's dark machinations appear to be rewarded, as a social worker assures her that she and Cade will remain together in the foster care system. However, as the social worker leaves to make arrangements, Emma gazes out the window with a chillingly victorious smile, hinting that her reign of terror is far from over.
