Emma Grossman, a nine-year-old girl living with her widowed father, David Grossman, is haunted by a tragic event. One day, she witnesses a cat drown in the fountain just below her bedroom window, which sparks a cascade of dark thoughts. Throughout breakfast, Emma shares her deep desire to earn a Citizenship medal, awarded to a student who truly embodies the values of her school.
As the highly anticipated "merit day" arrives, Emma is filled with hope and confidence, eagerly awaiting the announcement of the award. However, her expectations are shattered when Mrs. Ellis reveals that the medal belongs to her classmate, Milo Curtis. Struck by disbelief and disappointment, Emma masks her feelings until the celebration where she expresses her disdain for Milo to her father. Even though David scolds her for this bitterness, Emma secretly leads Milo away from the festivities and into a secluded area. Here, she coldly pushes him off a cliff, subsequently stealing the coveted medal and sneaking back to the party. Moments later, the shocking discovery of Milo's body sends waves of panic through the community, while Emma and David observe the frantic attempts to revive Milo from a distance, leaving his grieving mother utterly devastated.
In the wake of Milo's death, David seeks to understand Emma's emotional state, but she presents a facade of cheerfulness and detachment. His sister, Angela, who is a psychiatrist, believes that Emma is merely in shock and will process her grief in her own time. To complicate matters further, Chloe, the babysitter hired by David, becomes embroiled in Emma’s manipulative games. When Emma discovers Chloe stealing prescription medication, she leverages this knowledge to control her.
As the situation unfolds, Milo's funeral further exposes Emma's deceitful nature as she feigns sorrow in front of Milo’s grieving parents. Mrs. Curtis requests photographs of Milo, and David's inspection of these images reveals Emma’s lurking figure, casting a shadow over Milo's achievements.
Soon after, the school authorities, including Mrs. Ellis, arrive seeking answers, prompting Emma to eavesdrop. News about her last known whereabouts with Milo sends a chill through David, who grows increasingly paranoid about the implications. When Chloe uncovers the hidden medal beneath Emma’s bed, tensions escalate, leading Emma to orchestrate a dangerous scenario involving a wasp nest and the potential fates of those around her.
Emma's unnerving behavior becomes increasingly alarming, and David's concern prompts a visit to a child psychiatrist, Dr. March. There, Emma extends her deceit, sugar-coating her actions to appear perfectly normal. Meanwhile, Chloe's playful taunts about her aspirations to become David's partner push Emma over the edge, and she resolves to dispose of Chloe in a horrifying turn of events by setting the shed ablaze.
As the chaos deepens, David grapples with the shocking reality of Emma's admissions regarding her violent deeds. Heartbroken and desperate to protect her from an institution, he takes Emma to a remote lake house while attempting to devise a way to end their ordeal. Unbeknownst to him, Emma is plotting against him, triggering a harrowing series of attempts on his life.
In a moment of desperation and self-preservation, David tries to take control, but Emma’s cunning prevails. By manipulating circumstances and drawing in the caretaker, the confrontation escalates, leading to a catastrophic end for David. As the townspeople arrive to witness the aftermath, Emma remains eerily calm, embracing her aunt Angela with a facade of sorrow, but with a chilling smirk that betrays her true, malevolent nature.
